If the client passes the server a token which the server can verify against the third-party service providing the login, I don't see a reason not to trust the client. I'm very interested to hear what kind of security problems this could bring - if they can be mitigated, using this library would be very convenient for some projects.

I didn't look at this in detail, but this appears to also be using refresh tokens. Getting clarity on refresh tokens is a bit tough, but they are intended to allow one to request a new access token when the access token expires. I don't think refresh tokens are intended to be stored client-side as they are with this. If a refresh token is compromised, it can be used to request an access token for the user.