Wash trading is not just one thing, I give you that. But a frequent way of achieving wash trading is to buy and sell from yourself via multiple accounts. KYC/AML + Terms of Conditions specifying you can only own one account prevents that, as much as it can at least. Even if you are two different people just trading between you, AML laws will prevent that and surely Coinbase has the most basic checks in place to detect something that simple.
I'm not entirely sure how KYC/AML are related to wash trading ...
In other words: how is the amount of checks they impose on their customers related to what goes on in their trading engine?
Or do you assume that because they're very strict on one thing necessarily implies they're strict everywhere?
That's quite a stretch.