I mean he literally says he learnt it from the Emperor fight.
Basically he had no clue what Doffy was talking about or it's another PIS moment from Oda.
Where did "the user" come from?
Vergo had better Haki than Law. Vergo could not withstand Law's cutting hax. Yonkos did though.
Clearly there's a threshold, it's just that it's super high for Law's.
And with Doflamingo's comments, it ain't that high for every DF user.
If Doffy literally told Law that Vergo could negate his df because vergo had better haki... WOULDN'T LAW ALREADY KNOW THAT FACT? So no need to "Learn it from the emperor fight". But he DID learn it in the emperor fight. ie, what doffy said was wrong.
Vergo had better haki by your own words, but still lost. So his haki wasn't enough. And Law says "because of their very stong haki" when asked why he can't move the yonko. And later, he says he learned from the yonko fight. ie, he used "very strong haki". Therefor, Law has comparable haki to the damn yonko.
You're now inventing stuff like haki thresh hold for each fruit lol. hax is hax. Doffy was wrong. Law now can use haki comparable to yonko. That's it.