They are
easily detectable if the developers have dedicated a part of their software on defending the user from such attacks. Bitcoin Core
has worked on it, for example. The user cannot be expected to use all sort of coins, from different devices, for the sake of confirming they aren't under sybil attack. I haven't found anything substantial in their client's repository.
I feel like we are talking about different things, sybil attacks in Coinjoins are pretty useless if you are not the coordinator, if you want to attack me you need to guess the exact round that i would be joining + stop others from joining the same round or in other words make every other participant (you) which I can't think how would it be possible.
If you are indeed the coordinator then you have got an edge, you could simply reject all other users in the round I am joining and the attack will work, but again, doing that would be obvious because other users who have valid inputs will be rejected for no good reason and that would indicate the coordinator is attempting the attack.
Sybil attacks on the p2p network are different, since other nodes won't care if your node is rejecting them or has gone offline, since there is no central cordinator the whole thing is different.