What is the physical meaning of coherence bandwidth in mmWave massive MIMO channel?
You are right, coherence bandwidth is the frequency domain counterpart of delay spread.

However, to achieve diversity across antennas the spacing between the antennas is important "relative to the environment".Let me explain that a bit more. In a user mobile phone you would typically find the order of wavelengths seperation between antennas sufficient to get independent fading accross them. However, the same is not true at the base station. Typically at the base station the seperation between the antennas has to be a 10s of order of wavelength. Why so? Because a user equipment near to ground sees a much richer scattering environment, which are enough to cause phase changes within a wavelength. However at the base station (usually placed high up in the air) the scattering environmental is not that rich and hence more separation is required.

So, to study the effect of independent fading with antenna separations one can use the methodology to see if other antennas are seeing the same coherence bandwidths or not. It is all part of channel profiling whenever new frequencies are deployed.