It's both.
He isn't as dominate as he once was but at the same time the gap at RG between him and the next closest guard is so wide that even with him having a "solid year" by his standards is still better than all of the other guards.
The "getting recognition based on past achievements or his name value" is a cope out for haters because them simply saying that means they don't have to answer the question of "okay name 2 right guards that should of clearly gotten the award above him" because reality is they can't answer that question.
They'll discredit Martin on the basis of "he only got in on past achievements" but won't say why or who should of truly gotten it
No cop out at all. You can generally look at any year end list like this and see several folks who's inclusion is very questionable with the only real explanation being "name recognition".
I do agree that he does get some advantage because he plays RG as there are fewer high end players on that side. I would still contend that Wyatt Teller & Shaq Mason are probably better at this point though. I didn't watch either extensively this year though.
Martin doesn't "lose" a lot but he also doesn't "win" a lot. Too much stalemating in run blocking for me to consider him the best. Too often I have watched teams easily handle the defensive line we faced and struggled against the previous week. He had a stretch early in the year where he looked to be dominant again. I assume age is preventing him from being able to sustain it though.