he possesses stronger swordsmanship, even if you don't think he's ultimately stronger than roger
But here is the thing, if roger wins anyway, you know it's because he's carried by fate. That concept alone can justify in this manga why someone who is for a fact inferior in combat can win.
Even if roger's haki is just that strong, what did joyboy level haki do? knock out vice admirals? how is that gonna help him beat mihawk, mihawk also doesn't rely on a df ability like admirals for example. Unless you think roger can completely overpower Mihawk with his attacks.
And if black blade = superior use of ryuou as well, in terms of just utilising haki, why can't that get the better of stronger base haki, if you believe rogers haki is > mihawks.
It's like oda is creating distinctions of ability only to have them rendered obsolete in that case. Go master swordsmanship to its pinnacle, create the most elite blade in existence, then lose to a stronger will anyway.
It would just be like katakuri vs luffy, katakuri is getting the better of luffy in the fight, but luffy refuses to go down until katakuri's stubbornness get's surpassed and kat gives up. Is that what would happen in roger vs mihawk?
I'd like to think in such a duel of dangerous powers roger can't afford to make many mistakes against someone who is superior to him in combat ability with a blade. But if he can is he just going to out-will mihawk till mihawk drops his sword? Or will lightning strike again