It's not obvious on the website but everything is on Github[0] with different repos for the forked Dolphin build, the desktop app, melee assembly...

As to dev blogs, they have a medium[1] with some interesting posts, but nothing too recent, I hope they'll do one in time with all the new stuff.

On top of the rollback, something very notable they did was realising melee has 3 frames of input lag when running on native hardware, so they managed to remove them from Dolphin when running on 120hz screens. This allowed them to keep those frames as a delay buffer without actually having a real effect on gameplay.[2]

[0] Slippi Github: https://github.com/project-slippi/

[1] Dev Blog: https://medium.com/project-slippi

[2] Post about Faster Melee, the previous netcode iteration, going into the input delays: https://www.reddit.com/r/SSBM/comments/azoiqz/faster_melee_2...