From @jsn.sherman:

TL;DR
We can add home wiki to the editor data we fetch from the globaluser api. It'll be an easy task.
Long rambling history of us looking at how to think about this:
we previously kept an attribute called 'home wiki' back when users had to pick a project as an id source for sso, but it didn't mean the same thing as the central auth home wiki. It just tracked which project the user signed in from. We dropped that user data from the platform when we moved to sso with meta, since it was no longer required for sign in.
We chose not to pull it in from the api, because there wasn't really documentation for it, but there was an SUL migration thread about how the home wiki determination could be somewhat arbitrary.
Today I took a look at the central auth code that determines the home wiki, and actually it's very reasonable. It tries to determine an actual primary account based on how the user came into central auth; if it can't do that, it just goes by highest edit count.
https://github.com/wikimedia/mediawiki-extensions-CentralAuth/blob/475e474bf08c0b44d14371bddfaf879c68ea9cc2/includes/CentralAuthUser.php#L734-L773