I’m not convinced. In the UK at least most floors are wooden boards over joists, I’m not sure of the exact measurements, but from memory about 7” x 1/2” x pretty long. A floor is conceptually not hugely different from a giant guitar soundboard.
One of the reasons I’m a big fan of a good carpet plus underlay plus rug situation is to apply as much damping to the floor as is possible, as in most cases it is an obvious resonator (the other reason being I also view it as the ‘dead end’ vs. the ceiling acoustically).
Anyone who has ever owned a suspended turntable with a high centre of gravity (Linn, TD-160 etc) will know this only too well as in most UK rooms you have to tiptoe around like a cat burglar in a sleeping household or the turntable will jump. The floor is often not a stable or stationary structure. It has a bounce, one or more resonant frequencies or points of movement.
Sure, the Earth’s gravity tends to dictate that speakers and their stands end up on the floor, but we should not necessarily view that as an ideal, especially when it comes to physically coupling them and transmitting energy and vibration.