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Neovim is a superset of vim, so this argument applies to both.

No, Neovim is not a superset of Vim. Vim has things that Neovim doesn't have and vice-versa.

Neovim remove set compatible (hardcodes set nocompatible). Vim support the POSIX vi via that option.

Not quite. Vim is vi compatible by default. You have to explicitly opt in for the newer behavior.