This is definitely the case. Since the layer key is at the thumb, it doesn't require moving your hand and you barely need to move your thumb. As somebody who suffers from wrist pain, this makes a huge difference.
Those are slower than fingers and can’t be used as fluidly, in particular when using them as modifiers for keys typed with the fingers with normal typing speed.
Drummers do it. It takes some training because the nerve to your foot is longer than the one to your hand, but with good audible feedback it can be done. (If your feet are a meter farther from your ears then the sound from them takes 3 ms longer to be heard. Nobody types that fast but I'd you wanted a truer signal you could generate one in software.)
Not sure about the drummer analogy, because we don’t type in a rhythm. Hands and feet can maintain the same rhythm, and that has the effect that their actions can coincide with high precision. For arhythmic actions, my experience is it’s more difficult to synchronize to the typical subsecond precision for typing. For example, drum your fingers on a table in sequence, and then try to replace one of the fingers with your foot. It is magnitudes slower. While training will improve it, I don’t think it will be able to match the finger drumming, unless you introduce a longer-running rhythm. Keyboard shortcuts are similar.