The "actually encrypted" part is NaCL (ED25519 + sha256) as supported by Go [2]. Interestingly, the common way to use NaCL applies Curve25519 to encrypt a symmetric key which is the used for the payload. They don't do that. AFAICT, everything is using the ECC curve.

[2] https://keybase.io/docs/crypto/kbfs

So they've rolled out their own encryption?