The fact that Sasaki is shocked at Yamato's transformation with an expression of "?!" lends me to believe that this is not just a DF, but a weird trait exclusive to Yamato which even the beast pirates were not aware of
Actually no. I mean if we're going with "Kaido is an actual dragon" theory.
Then all the beast pirates have seen Kaido transform many times.
What's interesting is that Maybe they don't actually know that Kaido doesn't have a df.
So if Yamato also transform into a dragon it would be shocking. Not because she transformed but because there can't be two devil fruits of the same thing
If Both Yamato and Kaido transform into the same type of dragon, that's explicit proof that these aren't devil fruits because two Devil fruits can't have the same power
I think that's why Oda has made sure that Momo doesn't transform into a dragon in front of the beast pirates. So that when Yamato does it, it's an exploit shock to the beast pirates