Kaido lifted Onigashima island without needing transformation:
Another example is Momo's durability increased without needing transformation:
This is Ancient not Mythical still Ulti's attack without transformation as we see from green color was actually imbued by her DF:
Marco uses his DF without transformation:
Saturn's eye beam ability was possibly related to his Mythical Zoan DF and he doesn't need transformation to use it:
So the excuses that ''Yamato didn't use DF vs Ace'' completely baseless.
First of all, the fight was mostly
offpaneled:
So its not clear if she used transformation or not.
Otherwise it can also be said that Ace was
not using his Logia vs Yamato:
So there is no clue Yamato didn't use her transformation, for example she randomly
turned off the Mythical transformation vs Kaido:
That doesn't mean Yamato's physical stats, stamina, durability aren't boosted here by her DF.
And even if we assume that she didn't transform, its basically confirmed at this point Myhtical Zoans don't need transformations to use their power and they can boost their physical stats stamina and durability (see the other examples above from Kaido and Momo).
Another excuse:
''Yamato had Kairoseki chains''
Yamato's chains were definitely not Kairoseki, Luffy touches it casually and he doesn't feel weakened:
When Luffy touches Kairoseki:
Luffy couldn't even use Haki while touching Kairoseki:
So she definitely used her DF as we know she
doesn't need any transformations to boost her physical stats, stamina, durability and other abilities as a Mythical Zoan (see the Kaido and Momo example above) and also it was a offpanel fight so we didn't even see she transform or not.