if the window frames are recessed in the floor by the frame widening,
They are not: normally the window stands on the floor on the inside, on the outside the cover is in front of the floor structure. Here, apparently two more frame profiles are supposed to be doubled at the bottom, so the entire window stands in front of the inner floor and on the raw subfloor outside. Neither is recessed anywhere, why should it be?
I could imagine that the vibrations could be reduced.
It is sufficient for the frame that its profile is made with normal thickness all around. Tripling the underside makes no sense.
The only thing that helps against oscillation of vibrations is stiffening in the profile – and for 2 x 2 m or even larger fixed panes a central mullion.