The reasoning behind this is complicated. Alex doesn't lie (or at least, not very well). We're not sure if he's got the theory of mind to understand that we don't know everything that he knows (ie, that deception is possible).
However, that does not mean that he can't be mistaken. He's often announced "You did a poo" when there was nothing in the toilet or something which dropped out of his underwear. The statement might be eagerness to claim the rewards and evidence that he doesn't quite understand the whole toileting concept either. He might not understand the difference between a fart and actual production, for example.
In order for his toileting skills to progress, we need to make sure that every reward is given only to qualifying production. One or two sketchy choices could undermine the painstaking progress up to this point.
This illustrates how differently Alex's mind works from the rest of ours. If he is still in the universal knowledge stage of mental development, then he can't understand that myself, Dave and Nathan (or anyone else) is actually a separate person with limited perception. He would get frustrated when we don't automatically understand what he wants (which we've seen some signs of) and the verbal dance we make him go through "I want
When a mind is in universal knowledge mode, teaching is difficult. How can any information be coming in from outside? There is no outside. This may explain why the most successful method for teaching Alex is to break things down into very small steps and then give him opportunities to successfully do those steps. Then those small successes can be put together into new skills.
I see a lot of assumptions that everyone with autism is effectively neurotypical, just with a blocking layer of perception and behaviour. And for some children, this may be true. But for others, we are dealing with a fundamentally different way of perceiving and understanding the world around them. We can't assume they're desperately trying to communicate through the blockage or that if we could simply find a way around the blockage, everything would be fine. Rather, we have to find a way to bridge the gap and understand that it could be a lifelong effort at connection, all from our end.