Yes, federated login methods use the metadata from the IdP for the creation of the user account.
I’d be extremely hesitant to recommend a change in core (especially in the sensitive paths) for such a niche use case.
For you, I’d recommend setting up multiple browser profiles, each tied to a different ID account. Then you can use those to quickly log into try.discourse.org.
I do something similar; each profile is tied to a different email on the backend that ends up in the same inbox.
To help support this (and other shenanigans), I have a particular mail domain that rewrites
LOCALPART@SUBDOMAIN.mydomain.com
to
realemailaddress+LOCALPART%SUBDOMAIN@mydomain.com
(or, depending)
realemailaddress+LOCALPART@mydomain.com
If you didn’t want to do such a setup, I’d recommend creating a JS browser bookmark snippet that submitted the email change request for you once the account was created.