Bro i know some zorofans dont read the manga but it was baratie that Mihawk said he doesn't use canons to hunt rabbits.
He didn't tank that bullet, he used Yoru to deflect that bullet, not even his dagger. So even if would grant you the "haki didn't exist yet" point it doesn't make sense, it was threat to him
Lucky Roux is acing Mihawkblud if he lands a headshot lets not kid ourselves.
Also straight up made a haki headcanon feat again after the week where a hakiless character one shot one with haki. Did oda not invent haki last week?
This "haki makes bullet useless" didn't appear once in the manga just posted by powerscalers