I actually don't know. Oda has been very weird with Zoans. Even on the verge of death, Jack refused to transform into his Hybrid form and would have died had Kaido not intervened.
This makes me believe that Oda is not willing to have Zoans use certain forms before he's ready.
This makes me sceptical that if Yamato had a Devil Fruit, she used it against Ace.
Both at the beginning and end of their battle, Yamato was only shown in Base. It makes little sense to transform into Base then Hybrid then back to Base.
It's not as if the fight was entirely offpaneled. We got several pages of it:
There are a few short timeskips during the battle and we see Yamato not transformed every time.
So I think it's the case that Yamato did not actually use her Zoan.
Like the argument that Yamato used her Zoan off panel is not really credible given that we're shown several panels of different stages of the fight.
As for whether Base Yamato = Ace should apply to Hybrid Yamato and Ace, I would ask your opinion on the Dukes. Does Dukes ~ Base Jack imply that Hybrid Jack ~ Dukes? If you disagree, then I think the same standard should be applied to Yamato and Ace.
Lastly, we don't know that Yamato had her DF four years ago. She may well have acquired it after her fight with Ace.
As for Linlin vs Kaido, Linlin was also nerfed
- She didn't use Prometheus.
- She didn't fuse with Prometheus.
- She didn't use Zeus.
A holding back Linlin stalemated a holding back Kaido.
In contrast, a holding back (or pre Devil Fruit) Yamato stalemated an all out Ace.