I'm worried that the provided information could be incorrect. For example, that user could have messages waiting to be delivered to himself. In that case, I think signal doesn't know the senders but should still disclose the number of those messages and their size.

Signal erases that kind of information but I'm pretty sure that user must have had some messages delivered to them while signal was processing the subpoena. So pretenting they don't know anything else is just wrong IMO.