Good morning.

I have found this point impossible to ignore. You are misrepresenting the argument.

Sybil attacks can be used against IP anonymization layer such as I2P.

I assume you have made this mix-up, because these systems are called mix network. It sounds similar to the mixin parameter of Monero.

To compromise ring signatures with such an attack, an attacker needs to serve you a completely changed block chain that contains only their own signatures.

If an attacker can do this, you have lost and it does not matter what coin you are using.