So, suppose a person has a choice between Captain Crunch and Grape Nuts. They choose Captain Crunch because they like Captain Crunch and hate Grape Nuts. Maybe they even acknowledge that their aversion to Grape Nuts was caused by a traumatic Grape Nut experience in childhood. This person might say "I'd never choose Grape Nuts! Under no circumstances would I choose Grape Nuts." But, if they're a normal person, they would still say that they could have chosen Grape Nuts. But, if under no circumstances would they have chosen Grape Nuts then how could they have chosen Grape Nuts? The two things are in conflict. See? Well, they could have chosen Grape Nuts because there's an implicit "if I wanted to!" If they wanted to, they could have chosen Grape Nuts. Get it?

But the denier interprets "could have" literally and says "well if you rewind time things will proceed exactly the way they did the first time." But suppose we're in a universe where you could rewind time and things not proceed the same way as the first time. We do rewind time and this time our person chooses Grape Nuts. Now, the free will denier will think "Hmm, maybe free will is possible in this universe?" Except that this result is completely at odds with the common notion of free will! Why would I ever choose Grape Nuts if I hate them? The denier has things completely backwards. Situations unlike demonstrations of free will suggest to him that free will could be possible 🤣

Now, the denier will say "Well, just because a situation allows a person to have done otherwise doesn't mean they would have!" But if the "could have done otherwise" never comes into play then what does it have to do with choice? The answer is nothing.

Having done otherwise makes more and more sense the less strongly one feels about a choice. But the less strongly one feels about a choice, the less of a "choice" it is and the more it's just closing your eyes, spinning a globe, and randomly pointing at a country. The only time a "could have done otherwise" opportunity would come into play is for something that wasn't a choice at all.

TL;DR deniers don't understand "choice" or what "could have done otherwise" means. take it literally which is absurd. but but muh predetermined, but muh determinism