Seems a rather thin distinction.
Plenty of people who have served in the Legion have seen and described themselves as mercenaries.
And the same Gurkhas who serve the British probably would have turned around and served the Indians or whoever else (Singapore, etc.) was recruiting, had they not made the cut for the British service.
It is a real distinction, and a legal one, and few mercenaries become members of the Legion, mercenaries come out of armies, they don’t go into armies, speaking in terms of the traditional white, or Western mercenaries.
In Africa during the 70s and 80s, you could find work as a mercenary, or you could join the Rhodesian or South African army, two different things.