That is all they claim to store for later retrival*, which is not the same as what they would be able to capture in real-time and hand over to law-enforcement if forced to by a court (and they wouldn’t be able to tell you about it because of a gag-order).

*However this claim is contradicted by the source-code of their server (which they sometimes publish) which seems to store significantly more and of course this is assuming the code is indeed the same as the one they run on their servers, which is unclear.

Edit: and their servers run on AWS, so even if Signal itself doesn’t store the IP addresses, Amazon certainly could.

You seem to be highly misinformed what metadata is. A server for example will always have access to unencrypted IP addresses from the clients connecting to it, this is impossible to avoid unless you use a service like Tor that relays internet traffic, but that has very little to do with “encryption”.

Except that most of the metadata isn’t encrypted anywhere and usually also can’t be encrypted as otherwise the service wouldn’t be able to function.

This is false. There is still plenty of metadata that centralized providers can capture and turn over and that is usually sufficient for law-enforcement to get the information they want.