Fair enough, git clean does exist. However, if the button saying "discard all changes" is really a button that runs git clean, that's just a plain terrible design choice. git clean is "delete all untracked files", which is specifically not discarding changes, because there can be no changes to discard on an untracked file. Even talking about "changes" to an untracked file in VC context makes little sense, because the VC system doesn't know anything about any changes to the file, only whether it exists or not.

That's not even mentioning the fact that the option to "git clean" shows up as one of the easily accessible options in relation to a staging process. Especially if you're coming from the git CLI, you're likely to associate "discard changes" with "git restore".