Flarum is already using such a library, though not that exact one. It's using the League OAuth client http://oauth2-client.thephpleague.com/ Which could be described exactly as the one you suggest.
I suppose using HybridAuth (via an extension) would require digging deeper into Flarum events, as Flarum auth extensibility is currently expected to be done through league-compatible clients. For example the Flagrow Telegram extensions creates a custom League+Flarum compliant driver for Telegram. And you might not get much of an advantage as most popular services already exist as League-compatible drivers.
If you're suggesting switching from League to HybridAuth in Flarum core I suggest you open an issue on GitHub so it can be tracked ?