I think they mean the commit message editor, which git will use to open a temp file to save the message to if you don't specify a message in-line with the "-m" flag when committing, including when a merge commit is initiated by a "pull". This happens on the CLI, it's just usually (though doesn't have to be!) a command line editor that it opens. I think vim's a common default.

AFAIK whatever's opened does need to block the CLI, so you can't use a command that opens a GUI editor then returns immediately or git will interpret that as your having closed the file without saving, but otherwise any editor should work, CLI or GUI, and can be assigned in your git config.

In a thread about common sources of confusion, I think it would be more helpful to leave the misunderstanding so others might learn from it. Ie, edit to add "this is a misunderstanding" to the top, not replace it entirely.

Git invokes an editor, for writing commit messages, etc. (it looks in the VISUAL and EDITOR env vars). That could be a GUI text editor, or something running in the CLI (personally, I use emacsclient to open a new buffer in an existing Emacs window)

What they're saying is: if you quit that editor without saving the commit message, git will abort.

You can also quit the CLI editor, e.g. vim.