ok fair enough, sorry i may have misinterpreted what you meant.

it sounds like your argument is that if the attacker doesn't know the service is running then the assertion that this reduces the risk profile is classified as an obscurity control - this argument is correct under these conditions.

however, certain knocking configurations are not obscurity, because their purpose & value does not depend on the hope that the attacker is unaware of the service's existence but rather to reduce the attacker's window of access to the service with a type of out of band whitelisting. by limiting the attacker's access to the service you are reducing the attack surface.

you can imagine it like a stack call trace, the deeper into the trace you go, every single instruction represents the attack surface getting larger and larger. the earlier in the trace you limit access to the attacker, you are by definition reducing the attack surface.

in case i've misinterpreted what you meant. susceptibility to a replay attack does not mean something isn't a security measure. it means it's a security measure with a vulnerability. ofc replay attacks in knocking is a well known problem addressed long ago.

perhaps the other source of miscommunication is for us to remember that security is about layers, because no single layer is ever going to be perfect.