With the kind assistance of X11 Quartz background app I have an X11 app Dia diagramming tool running.
It is enticing to see those mnemonics apparently (?) available to use. I have a serious beef with macOS for not helping those of us who are keyboardists to have an option to work in that manner.
So, is it possible to access those mnemonics? I have tried various combinations of Option, Control, Control + Option, with/without Command plus the given letter.