I disagree that that warning is reasonably clear. Even the comment that included it has the line of thought, where the user, not knowing what terms git uses thinks that they just did an action that is going to change each of their files. It makes sense that they'd want to discard those changes. That user then goes on with some snark about not wanting to learn any more about what they are playing with and that other programs would do the same, but "discard changes" seems like it would have a clear meaning to someone who doesn't know git.

The warning says it isn't undoable but also doesn't clarify that the files themselves are the changes. Should probably have a special case for if someone hits discard changes on a brand new repository with no files ever checked in and hits discard on a large number of files instead of checking them in. Even a "(This deletes all of the local files!)" would make it clear enough to say what the warning is really about.