I think what the token dialect refers to is whether or not the token is an opaque token or a JWT. I agree, that is confusing. Token dialect is not something I have seen referred to often.
At this point, I am fully aware of your preference for firebase.
As the feature says,
“Add Permissions in the Access Token”
As for why the AT is opaque, please see the linked doc above.
Thanks for taking the time to do that!