This is an implementation of Secure Remote Password (SRP) from RFCs 2945 and 5054. However its hashing and padding scheme differs from those and is is not inter-operable with them.
This was initially developed as part of the authentication process using in 1Password, and as such it includes code specific to that key derivation process for the client derivation of the SRP x.
The user is free to use whatever KDF they prefer, but it should be recognized that the SRP verifier v is like a password hash with respect to cracking. Therefore the KDF used to derive x should at the very least must be properly salted and should use a "slow hash".