Since we can now just click outside of the menu to get rid of it, having this extra entry is probably redundant. Removing it would be consistent with other pop-up dialogues.
It needs to be keyboard accessible as well though. But I think you can hit escape.
Tested to see if it was keyboard and mouse accessible on macOS 10.15, Windows 10 and Linux. It works fine. Tested by pressing ESC and clicking outside with mouse.
Tested with native client version 6364 and it looks good, i.e. the "Dismiss menu" is gone.
Looks good. It's still there in TigerVNC's java viewer, but who cares.