Perhaps it was poor judgement of him to try and convey that in a single sentence twitter post, in fact even using twitter at all.

That's nitpicking. We know they know who fucked kids, and regardless, they said they had the "client list" and were going to release it, so semantics are irrelevant at this point.

for someone who's all his deal is being able to write good, that sucked.

like a lot.

definitely didn't imply that there isn't a file named "Epstein's client list. docx", but there's no doubt that there's info about all his clients.

It's just strange that, despite criticising Trump constantly, he'd choose this specific thing to defend him on.

He could have said nothing.
He could have reasonably assumed that "client list" is short hand for data on who the clients are; a list formed posteriori by the investigators.

But instead he said "people who think there is a way to learn the identity of the clients are crazy".
For a successful author, such poor grasp on the language is suspicious.