When offloading a hook implementation to another function, which function should be documented as "Implements..."?
TL;DR: Yes, it's fine and widespread too!

It might look kinda unorthodox but it's just offloading the bulk of the hook implementation to an include to keep the main .module file smaller.  Here's where it's happening: menu_block.module (view source at the bottom).

A lot of other contribs do this too, for example: Display Suite (see line 48 onwards) and I'm pretty sure Views and a bunch of others too.