Considering they chill most of the time in their room and let the marines do the work..
Doesn't mean They seen their Transformations, but They probably know They are ruled by Gigantic Powerful Yokai
For example, those who were in Wano don't need to have seen Kaidou's Dragon Form before to recognize him when He appears as one
It's just enough to know that He is a Massive Azure Dragon
Ryokugyu met Kaidou before? No
But He knew the guy is a Massive Snake-Like Blue Dragon
Did you see any Vice Admiral question Saturn joining the Battlefield?
No, they simply said that those below Rear Admirals shouldn't stay near Saturn
Also CoO Users can recognize whom Haki They are detecting belongs to
It's not like Fujitora for example doesn't know which Haki belongs to who
So Lucci either knows that Gorosei can Transform, but witnessed it now for first time
Or He is familiar with their Haki & recognize who He is talking to