My understanding/speculation from that email Odell shared is that their trust/custody provider could deny access, or add lots of friction if an account sent coins to a coinjoin and Swan is probably trying to warn people ahead of time to prevent the headache.

I recommend multiple hops before coinjoining withdraws from US exchanges, and prior to sending to US exchanges for those wanting to buy dollars or other shitcoin under duress. The ricochet service performs multiple hops in an automated way to assist in this.

Ideally people would stop sending back to exchanges and buy/sell dollars p2p.