They're definitely sliding.
Never seen anything like it.
At 1:37 the slim guy at the back & woman in the hat at the front can be used as 'markers' for the guy with fair curly hair & moustache. Also check the slim guy at the back against the candles They change positions in a way that cannot be explained by perspective zoom or a simple tracking shot.
Then at 2:08 it's clear as day; you can even see the inertia as they set off. You can see a couple of the SAs, camera left, affecting slight covering moves to try hide it as they set off; changing eye line, point of interest etc
Same for the final move of the pair of rows, you can see start & stop inertia.
The two rows also move slightly out of sync, catching up & falling behind relative to one another, which becomes more noticeable as the camera tracking comes to a halt.
They were being pushed/dragged.
Best guess is they rigged up a track, same as you would for a camera, & mounted the seating on it - that's handy stuff to have around on set, it's not like they'd have to look hard to find something that can do smooth straight line moves on a movie set ;)
Camera track is very very smooth, but though a camera dolly weighs a heck of a lot [it's a 2-man lift just to drop it the 2" off the end of the track], it's nothing like 6 or 7 people to try to get moving without being able to spot it.
Note you never really see the floor, except for the barest glimpse, over towards the pink dressing screen [where none of the moves take place]
The camera is tracking too, to increase the sense of dislocation.
Oh... as to why... I've not the faintest idea - maybe the director was trying to inject some interest & movement into what would otherwise be a very tedious scene.
I'm not sure it really worked. Getting the camera moves smoother might have helped, even without the audience jiggling around in the background.