I think I learned about it back in 2018 when I wrote a python script to generate vanity IDs, which was really just a rewrite of a similar vanity address generator I'd written for the NXT crypto currency (now ARDOR), that uses curve25519 rather than ed25519.

I believe that the idea of concatenating the private key proper and the public key into a so called private key, is so that you don't have to calculate the public key repeatedly. Instead you just separate the ‘private key’ in two halves, and there you have it: a 256 bits private key proper, which is really just 256 bits of entropy, and an equally sized public key that is calculated using elliptic curve maths.

I can't be arsed to find the posts now, but I know I have written about it several times before when the talks have been about backup of ~/.ssb/secret file: using Samir's Secret Sharing Scheme (is that what it's called?) to backup just 32 bytes is much more convenient than backing up 64 bytes or more.

The concept is being used in ssb-{export,import,vanitygen} from ssb-utils from your's truly: https://github.com/kseistrup/ssb-utils (hopelessly unmaintained and outdated, so take it as a proof of concept only).