I tried DS1 in the summer so he could go pants-free in the garden etc. He was 2.5, and did not have a clue. He couldn't talk in sentences- just pointed grunted and used minimal words to make his point. That was the summer of the "two word picnic" when he dragged a chair across the room, pocketed some boxes of rasins from the high cupboard, came to me, said "walk", we went for a walk, he led the way, said "sit" and shared the rasins for us to eat.
He wasn't at the theshold for SALT, that came a year later.
We tried again at 3 and he got using the toilet reliably within a few weeks.
The autism diagnosis came at 9, and with hindsight, he isn't great at managing intericeptive signals so no wonder 2.5 was too soon for him. But at the age of toilet training, his additional needs were not obvious although there were traits that were present and relevant.
I'm not sure that 18m of strapping him into a "potty-chair" like the one I was trained with would have been that productive, despite saving laundering 18m worth of food allergy reusable nappies (disposables were useless).
At least being 3, we got straight on to using the toilet and cut unnecessary transitions which with hindsight could have been awkward.
DS2 was ready before 2.5 and didn't want to wear nappies any more. The awkward bit was him deciding while camping abroad when it really wasn't practical! He was sorted in a couple of weeks. Much simpler than DS1 and he could communicate much more effectively too.
Shoelaces are an on-going work in progress for them both... and both have grown out of velcro shoes. (Dyspraxia, dyslexia)