It looks like they are using Coinfirm API to get information about a particular Bitcoin address.
Absolutely. And nice for the CEO to spell out in the thread I linked above just exactly what Coinfirm are doing to your UTXOs you try to mix with Wasabi. Firstly they link your UTXOs with off-chain information, entities, identities, etc. Then they cluster your UTXO with others belonging to you, examine your transactions in order to identify patterns which can be used to link to your other transactions, look for anomalies, graph analysis, throw in some machine learning and AI as well. That's exactly what I want to happen every time I try to coinjoin!

But of course, the specifics of their techniques remain completely secret and they can refuse your coins for any reason and not tell you why. Super transparent!