Google was the only way into the dashboard, which is an odd requirement for a build cache: the account most people have open while wiring one up is GitHub. It is now offered alongside Google on the same screen.
Signing in with GitHub reaches an existing Cachely account only when GitHub reports the same email address as verified. An unverified address is refused rather than trusted, so a second signup cannot arrive at a workspace that belongs to someone else, and the address is matched literally - no normalising that would let two different people look like one.