Worstgen
Light D Lamperouge
Correct me if I am wrong, this is the first time we have seen Roger with a sword, disregarding the hype up chapter for Shiki I think, and this was Roger at the start of his journey, he could have made it such during his later adventures when he became stronger. Secondly, I have never argued that just having a black blade means that you are stronger by default, Zoro has had one since before the TS.
Light D Lamperouge
Would you have placed that Zoro above Shanks, Fujitora, etc? Absolutely no. Thirdly, what I think many people do not realize is that Roger was/should not be the best at everything, every top tier has something that sets him apart from others, WB's DC, Kaido/BM dura, Akainu leathality, Kizaru presumably speed, Aokiji hax.
Light D Lamperouge
Imagine if now Roger had the same dura as Kaido/Bm, DC as WB, etc, there would be no one to rival him, but we know there were WB and Garp. So it can mean that Mihawk and Ryuma had better COA than Roger, while Roger also had top tier COA just weaker than them in that department. Zoro is hinted at making a black blade in wano/ post wano and will at best be a low top tier, still does not mean he will be stronger than
Light D Lamperouge
Shanks Mihawk Fuji, or other swordsmen, or other top tiers. He will have to level up his other stats to top tier, and find one in which he excels, and that could be COA. Plus even if he makes it a black blade, some of it should presumably go to Oden and his haki as well, since if Zoro only makes Enma black and not his other swords it means that enma probably experienced haki coating a lot more than them.
Light D Lamperouge
My point is that Roger does not have to be the absolute strongest in everything, hence him having weaker coa than Mihawk and Ryuma is plausible, as he has weaker dura, presumably DC, hax etc than other top tiers who excel in the aforementioned things. There a lot of ways to explain that, like I wrote he could have made it like that later, he could have switched his style, like BM, he could have added something new.