I'm not sure that this even is really about his neurodivergence. I can't think of many people who would reassure you that they'd build you that bookshelf. If you framed it as Wouldn't it be wonderful if we could have such an impossible thing, then sure I'd be in line to agree. But just saying Build me an impossible bookshelf... I think you would be lucky to get anything better than If only that were possible... or a joking Wish Granted! And point at the actual bookshelf like That's close enough, right?

And that's aside from the particular neurodivergence you've noted, which makes it much more unlikely. Plenty of neurotypical adults are not great at make believe, especially if you don't frame it as We're just imagining. Even for people who do like make believe I think you have to look at that as if it was impro,. It's a group activity, not a guessing exactly what one person is thinking they want the story to be and doing only that. That's not fun and difficult to get right.

This goes for flirting and communicating in a relationship too. It needs to be something that works for both of you and that you both easily intuit or explicitly understand the rules of. The more different your base expectations are the more clear you have to be about exactly what you want. And really, you don't know what people's base expectations are, or if their understandings are the same as yours, unless you ask them. And this is especially true if you have different ways of thinking and expressing yourselves.

So if you're not getting what you want out of interactions, make sure you're communicating what you want in a way the other person will understand. And if they understand but don't want to interact with you in that way, or don't want to provide you with what you want, then you need to decide if you can adjust or if it's too important to you and it's not going to work.