https://forum.z.cash/t/what-does-the-zcash-team-think-of-monero/1465with ring sigs you have a specific set of 3 to 10 possible sources of funds, which is certainly better than having just 1. if you make a ring sig tx, then your signature is one of the possibles, since it is your transaction.
So for a tx with mixin of 3 this is what an external party sees:
[yours] [otherB] [otherC]
[otherA] [yours] [otherC]
[otherA] [otherB] [yours]
So while it is true there is no certainity that it is yours, one third probability says that it is.
For zcash this is what the external party sees:
[X]
All that is known is that somebody did a zcash transaction.
So for anybody that says that ring sigs and zcash are even in the same league of privacy, they are missing the simple fact that each ring sig tx you do, your sig is part of the tx. Maybe there is another layer on top that tries to hide this correlation, but on the blockchain, there is this finite and positive correlation.