The MACO's are space marines simply by the role they play. No, they're not technically US Marines in universe, but out of universe they are based on US Marines. There's no need to get into complex definitions. Marines have existed since the days of Pharaoh. The NX-01 was based on the 21st century US Navy. The MACO's were based on the 21st century US Marine Corps. Especially, the uniform. It's so similar to the Marine Corps utilities rolled out in 2002 which were totally unique at the time.
Even if they weren't serving on the NX, they're still sort of fulfilling the role of space Marines by guarding offworld colonies or whatever it is that they do.
Even if they weren't serving on the NX, they're still sort of fulfilling the role of space Marines by guarding offworld colonies or whatever it is that they do.