(not the op)

The problem with coinjoin (and monero) is if someone knows where you spent money to, and is trying to determine if you were the receipient of a source of funds, they can just look at the chain and see there is an on-chain link between the two UTXOs. Far from 100% of coins out there have been coinjoined. So if that link shows up, there's still a decent chance the connection is a real one and not coincidence.

Coinjoin is good at ensuring that people sending you money, and people receiving your money, can't determining much from that information alone. But there are cases where you need better privacy than that.

Lightning is good for completely eliminating the on-chain links. As long as the adversary doesn't have information on what transactions happened in the chain of Lightning nodes you used, they just don't have the information necessary to make that linkage. But Lightning isn't perfect either. Bad actors like chainalysis agencies can try to monitor LN channel balances with probing attacks. And those bad actors might be running nodes too.

But that's always true of privacy tech. So layer protections and don't rely on any one thing!