Also, you’ve gotten the secret off the client machine, but the attacker can still do anything the secret can do by using the proxy? Perhaps I’m missing something.
The attacker can currently do anything with the secret by interacting with the sites allowlisted for that secret, but they can't exfiltrate the secret, which is the goal of this security control. You can do better, if you like, by further locking down which endpoints they can call, but the wins past "log carefully and no exfiltration" get smaller and smaller, and at some point you're burning time that can be spent more productively on unrelated controls.
If you get what it's doing, you get it. :)
Not blocked necessarily, but if they want to leverage a stolen token, they’re now forced down a more difficult and highly visible pathway.
You can imagine anomaly detection along the lines if “hey your rails app just made a type of request that it has never made before”, but even just monitoring the metrics of the proxy could tip you off if something is going on.