I don't see you mentioning anything that sounds at all like a bug...did you get one from them before you researched Replaceable Parts?
Militaristic city states giving away unique units from a civilization that's not in-game is definitely intended, as the tool-tip states, it should be necessary for you/them to research the tech required for the unit to be built before you receive them.
That is, it's usually been a bit fuzzy as to exactly when city states research technologies. If you're lagging, you'll sometimes get units you can't build from a militaristic city state (or resources you can't use), and if you're ahead, you at least used to have to wait for your city state allies to catch up, so the Foreign Legion could show up slightly early or late.
I read the tool-tip as suggesting that Firaxis has changed things so that your allied city states immediately gain the benefit of technologies you research, so that, if you get that tech ahead of the curve, you could immediately get your ally's special unit...but you may still be able to get it early if you're behind the tech curve, since I've definitely still gotten non-unique units that I haven't teched to when I'm behind the curve.