Phoenix just wants to be sure you don't use a seed with them and also other wallets. You can use a BIP85 wallet just fine, just don't use it elsewhere.
Phoenix is heavily integrated with ACINQ's node where other wallets are not. Having other transactions happen or channels being opened or activity that it's not expecting to see would break things, I imagine.
Phoenix just wants to be sure you don't use a seed with them and also other wallets. You can use a BIP85 wallet just fine, just don't use it elsewhere.