He means that rotates relative to the spindle. If you pay attention in your picture you see the upper die support being a bit tilted one side, that is because it's free to move, as it couldn't be otherwise. It's pointless to push sellers for much technical data, they are sellers not watchmakers.
Once again I suggest that you go for the other type, the price in the 1st link you included is robbery and the design is, after all, outdated.
So, asked the seller the same question and got the following reply:
"The upper die rotates when the spindle is turned. The lower die is fixed."
So, I guess this isn't an MKS 46610 copy after all, or the seller doesn't know what he is talking about. I did send the pictures to him. See if I can press him to select either picture A or picture B. Getting a decent press for armoured UB glasses without spending a fortune is beginning to feel surreal...