I was under the impression it had to be fanfic-esque sourced to be properly Mary Sue.Is it properly Mary Sue ?
Well, the specific definition varies a bit depending on whom you ask. When the term is used to describe a group or race (like Space Marines or elves), it's usually meant as a derisive complaint that the group or race is consistently depicted as being superior (often vastly superior) to regular people in practically every relevant capacity with few if any truly significant deficiencies.
Technically, but 40k official fluff is practically the same. Space Marines are super human to the point of ridiculousness just a little bit too often. Its disappointing really, even toned done to nonmarysue levels they are still amazing and appropriate for the setting. The problem is that the writers and game designers seem to occasionally fanboy out, and you end up with what should be a good little yarn turning into an epic poem about the size of space Marine dangly bits. The Matt Ward era being a pretty good example of what happens when you let a dev go fanboy.
On top of that the IP has a lot of bad internal tropes it can't seem to shake off. One of my favorites being to wax on about an individual's tactical genius, only to always have his battle plan be a mid day frontal assault with bayonets.