How can we be sure that Wasabi is using Chaumian CoinJoin? Couldn't they be saying they are using Chaumian CoinJoin, but are using a different kind of coinjoin and recording which outputs belong to whom?
And if you have doubt from client side, you could
1. Debug the software (from source code) or monitor network activity (which is very complicated since Wasabi use Tor) to see what kind of data are sent to the server/coordinator.
2. Audit the source code