Hi anon! I’ll do my best.

ABA is a scientific method:

• the first step in ABA is the observation of the individual and their response to their environment in order to identify unwanted “target behaviours”
  
• the second step is the systematic use of behaviourism-based techniques (aversives, rewards, operant conditioning) in order to eliminate unwanted “target behaviours” and encourage wanted behaviours
  

ABA as a “treatment” for Autism:

• the basis of ABA is behaviourism and the medical model of disability, aka the deficit/illness/”broken baby” model
  
• ABA’s main focus is on the normalization of behaviour (ie. extinguishing stimming, table readiness, “quiet hands”, eye contact) rather than on adaptive learning or accommodation
  
• and the end goal of ABA is often for autistic children to be “indistinguishable from their [non-autistic] peers”
• falling in line with those values, ABA principles push for neuro-normative methods of expression, like talking, even when alternative or neuro-atypical methods of expression, like AAC, would be better for the autistic in question
  
• in ABA, consistent positive reinforcers/rewards and aversives/punishments are used to enforce behaviour— reinforcers being withheld until the wanted behaviour is performed, aversives being used when an unwanted behaviour is performed
  
• common reinforcers include edible treats (gummy bears, chips, cereal); praise and verbal or physical affection; tokens that can be exchanged for “privileges”; stickers or stamps (often on a behaviour chart); a piece of lego or of a puzzle (aka a component to something the autistic wishes to play or complete); access to a favourite toy or beloved object; break time or a moment of rest from the “therapy”; access to the autistic’s special interest; time spent engaged in a “preferred activity” (aka something fun, like going to the park or watching a DVD)
  
• an increasingly popular method of reinforcement in the ABA/compliance training world is the use of a clicker (a training device for animals that make a loud sharp “click” sound to indicate that the wanted action has been performed and that a treat or reward is forthcoming)
  
• common aversives include the removal of tokens, stamps, stickers, or desired play components (lego, puzzle pieces); the confiscation of favorite toys or beloved objects; the withholding of snacks, water breaks, or recess/break from the therapy; the prevention of engagement in the autistic’s special interest or preferred activities; the use of “taste aversives”, such as pickle juice, vinegar, hot sauce, wasabi, or any other “effective” edible substance (the taste aversive is applied to the autistic’s mouth through the use of a spray bottle, or a q-tip, cotton ball, or pacifier soaked in the substance of choice); the use of tactile aversives (some examples are styrofoam, glue, bar or liquid soap, a piece of carpeting or upholstery fabric, sandpaper, or whatever incites tactile defensiveness or distress in the autistic); and the withholding of praise and/or physical or verbal affection
  
• in DTT (Discrete Trial Training, which is considered a softer, kinder version of ABA) the therapist or practitioner will not look at, engage with, or respond to the autistic unless they perform the wanted behaviour, and any unwanted behaviours are ignored in the same way, or met with “passive resistance” from the therapist 
  
• ABA as a treatment for autism focuses in part (and often a large part) on compliance training— for a truly horrific example of compliance training, see here (warning for massive ableism, dehumanization of an autistic adult, infantilization of an autistic adult, the demonstration of compliance training/ABA on an autistic person)
  

What you describe in your post:

• does not sound or look or smell like ABA, so I’m going to go ahead and say it’s not ABA
  
• so while the speaker may have truly been an ABA therapist that follows ABA principles, they either didn’t give a very good explanation of ABA or they intentionally left a lot out to make it sound friendly and great
• in the case of your “nappy painting” example, that is a clever use of sensory tools/providing a good sensory diet for the autistic person;
  and/or a commendable application of AAC in order to provide a more effective method of communication for the autistic person and reduce their frustration
• but neither of those things are ABA
• I’m not sure how it happened, but somehow any attempt at all to teach an autistic child anything became known as “ABA”— this really shouldn’t be the case, since ABA is a distinct scientific method, not just any kind of therapy, and especially not just parenting
  
• the calling-everything-ABA thing may have arisen out of the fact that, in the USA, ABA is the only therapy that insurance companies will cover for autistic children, so a lot of not-ABA therapy gets called ABA therapy so it can be covered
• but seriously, what you describe is not ABA, but it is exactly what the parents of autistic kids should be doing— providing alternative communication methods, putting a good sensory diet in place, substituting unsafe sources of sensory input seeking for safer and more effective sensory input
  
• aka, parenting their children in a way that understands and accommodates their individual sensory seeking and communication needs

In conclusion:

• ABA is a distinct scientific method that arose out of the behaviourism movement and the medical model of disability; ABA employs the systematic use of operant conditioning, positive reinforcers, and aversives; and ABA is largely focused on behaviour normalization and “indistinguishability from peers”
• autistic children may benefit from some occupational therapy to help with sensory integration, or speech therapy, or play-based therapy to foster development at their own pace, but all of those things should be the frosting (or smaller part) of their life, and the main part of their life, the cake, should be composed of the same teaching, play, mentoring, and time together that non-disabled children receive from their parents
• and ultimately, in order to parent your autistic child well- to teach them to use AAC, or to provide sensory input catered to their brain’s needs, or to grow them up into healthy, self-confident autistic people- ABA is completely unnecessary, and often even detrimental to those goals

That ended up being longer than I intended, but honestly, this is the best I can do while still doing justice to the topic. I hope it helps.