It looks like they're just taking the entire eightcc compiler and stuffed into vim buffer memory.
No human hand-authored this giant pile of VimScript: https://raw.githubusercontent.com/rhysd/8cc.vim/master/autol...
The current code is essentially assembly instructions running as Vim script, this gotta be incredibly slow, probably not much faster than an x86 emulator written in Vim script:
https://raw.githubusercontent.com/rhysd/8cc.vim/master/autol...
- https://github.com/neovim/neovim/issues/170
- https://github.com/neovim/neovim/issues/387