Keychat use seed phrase, not nsec. 

 It's actually good it does not use the key used for posting, but it would be cool if we could publish a signed note that ties the normal npub to the keychat identity for discovery. If I added someone to Nostr, having an option to write to them without exchanging some contact information would be good. 

They can write me Nostr DM anyway.

But then this could also work for SimpleX and others, essentially rebuilding the identity part of keybase, which was quite a good idea. No need of web of trust if you already got a social reputation - a website, GitHub account, PGP key, Twitter account, ...

Nostr could be the thing that ties it up and does not rely on Keybase corporation which was acquired by Zoom and more or less stopped working on the project.