They have plans to follow it later. It's a single private key and single public key for now for simplicity

Keychat allows for a nym with nsec and seed phrase, not sure if their seed phrase follows bip39, i suppose it should

Yes just created a nym in keychat, used nsec and restore wallet, boom 💥 in

BIP39 is used to derive BIP85 child key. I think it is the only option if using a single backup and index based derived key sets including BIP85 32bit hex keys used as nsec. It is very convenient if proper key management is required.