You're talking about a clash between two people around the same tier, just like Marco and Big Mom were around the same tier
We don't know anything about Yamato's canine ability, if it's really a df, if she already had it, or if Ace was going all out. All what Oda wanted to portray there was that they were comparable, with Ace stopping the fight before it became too serious after realizing Yamato was a prisoner
And if current Yamato couldn't easily deal with Sasaki's group(albeit while protecting Momo) then it implies her growth rate was shit if she even grew at all considering that shortly afterwards, Ace went on to beat Jinbei(a veteran Shichibukai) who is about to fight Who's Who's entire squad, with commander Ace significantly stronger than the version of Ace who fought Jinbei