Bare vi does not have windows, visual selection mode, multiple level undo, color schemes, etc.
NVim wants to improve some defaults, which cannot be done if it is beholden to all legacy code. That being said, I still think this specific undo example is insane.
It does something using neovim's internal terminal mode rather than use the file descriptors it was called with. Most commonly this breaks when a command produces an interactive prompt or uses a pager.
