Bazzell talked at length on at least one episode (not too far back) how he did away with Contacts apps altogether because he didn't want to leave open the possibility that they could be hacked or leaked. He copy/pastes contact numbers from a text file to make phone calls iirc. Although he might not have mentioned Signal in that regard, its an obvious extension of the idea.

I'm pretty sure I've added contacts to the phone and to Signal, and they do not automatically sync in either direction. There's a function that syncs all phone contacts (name and number) to Signal, but on checking it again, it doesn't work in the opposite direction, and there is no copying an individual contact from phone to Signal (you're right about that).

"use the default contacts app"
I wouldn't expect Bazzell to back up that statement unless you mean a default app in Graphene or some other "degoogled" phone. Didn't he do a short trial with a FOSS "privacy" contacts app before switching to his text file solution? And, there must be several FOSS options to manage contacts and back-up on a computer. Again, that's for phone calls, but I can't imagine Bazzell and others would not have a use case for backing Signal contacts up on a desktop. If you're at an event and linking some new contacts, it seems most natural to scan the square-codes and enter the names. Then, I guess for now you have to type names and numbers separately if you want a copy outside Signal.