If Mihawk was someone who practiced swordsmanship religiously in isolation, I could see why he wouldn't be stronger than certain characters.
But the fact that you literally have to forge it over countless battles, and roger and wb couldn't forge their beloved blades into Kokuto despite being in there fair share of battles:
Tells me that Mihawk's relentless pursuit must have outpaced them by a enormous margin, and it's not even surprising because even now Roger and WB look like professional fighters who might fight maybe once a year at some major event due to politics or delays or some BS.
And even when they do fight, like at God valley, you can say "roger fought imu" but what does that really does using one divine departure do to contribute to forging a black blade?
Even when Roger had run into WB towards the end it had been years since the last time, WB wanted to consolidate a family and Roger needed to get to the One Piece, he finessed Big Mom and avoided fighting where he could, they weren't actively pursuing strength in combat in the same manner as Mihawk.
Maybe you can say they had to bloom during the major events in their career and ofc fighting a domi reversi xebec is one of those major events, but I genuinely think Mihawk has much more battle experience than both of them.
So it might be like this, being a pirate who wants to be involved in major politics and world events can lead you to extreme situations where you can bloom and your haki gets stronger as a result of surviving.
But actively pursuing swordsmanship like Mihawk leads to unparallelled mastery. Even if he doesn't care about the One Piece etc, whereas Zoro's main mission is making sure luffy achieves his dreams first and foremost. That leads him to extreme situations being on the MC's crew which is why he can progress extremely fast relatively compared to anyone, if you take out the obvious of this being a shonen manga and it needs to happen while they're this young.
Perhaps this is how Oda could explain a scenario in which mihawk is still weaker than roger, despite being a better combatant than him, because being involved in more extreme situations like god valley trumps even going out your way to fight everyone.
But even then, EVEN IF Mihawk's haki was weaker in base strength, why the hell shouldn't he be able to best roger in a 1v1 exchange with the best haki utilisation in swordsmanship, best skill, best combat experience and strongest blade in the verse, other than Roger being destined as a .D.
You would like to think the "Haki above all" statement is multi-faceted, like how haki is utilised being as important as haki strength. Which is what Zoro got chewed out for by scopper. So of course it is important.
These guys look primitive compared to Mihawk, who is portrayed as someone who can use brutality with finesse, Even shanks using divine departure is like him just fanboying over old-gen, like how a hardcore ufc fan would try to copy some old-school fighter even though it's obvious today's fighters have evolved since then.
Anyway it's obvious Mihawk > them, this is just entertaining how oda would justify the opposite.