What’s the point of this demonstration? Well this comb filtering can happen when recording a guitar amp too close to the floor or walls.
Even worse, if you’re mixing with a reflective high-backed chair or are too close to a reflective wall, it can drastically alter what you’re hearing. Now let’s see what happens when you bring something non-reflective up to the mic.
There is far less coloration and comb filtering in that example.
Of course you can also use this to your advantage. Some engineers, such as Steve Albini, like to put mics right on the floor when recording drums. Because the distance between the mic and floor are minimal, there is very little phase shift and comb filtering, all frequencies are boosted.
So I guess the point of this is: before you record, have a look around to see if there is anything that is going to interfere with getting the best sound.