is multipath the real cause of small-scale fading?
If everything in the room was perfectly stationary, and only a single frequency was transmitted and received, then its amplitude would be constant despite being affected by multipath. The only way you would know that multipath interference was occurring would be if the received amplitude was greater or less than expected.

However, unless the room is cooled down to absolute zero everything will be moving (even if only microscopically). The higher the frequency, the stiller you have to keep everything to create a stable interference pattern. Take away multipath and you need a much larger amount of movement to cause significant fading - but then it is a large-scale effect, not small-scale. So since movement is always there, and small-scale fading doesn't occur without multipath, how can we say that movement is the cause of small-scale fading?   

Multipath can also cause fading effects even without movement. A practical device doesn't just send a single unchanging frequency and amplitude, but may spread its signal out over a wide bandwidth and/or hop between several different carrier frequencies. Different frequencies create different interference patterns which may be affected more or less by absorption/reflection of the room walls. The result is fading caused by multipath acting on the signal's frequency and modulation characteristics.