>>ZiiS+cd
The secret token is hashed first, and it's the hash that is looked up in storage. In this arrangement, an attacker cannot use timing to determine the correct value byte-by-byte, because any change to the secret token is expected to randomize the whole hash. So, timing-safe equality is not needed.
That said, if you have spotted a place in the code where you believe there is such a vulnerability, please do report it. Disclosure guidelines are at: https://github.com/cloudflare/workers-oauth-provider/blob/ma...