Microdynamics is obviously a thing, in that a musician can play notes with varying strength. It's less clear how this would apply to speaker performance, and I think this is the core of the disagreement. Any speaker is clearly able to produce a high sound pressure at one instance, and a lower pressure right afterwards, otherwise it wouldn't be able to produce any sound at all. And those high and low pressures need to be rather close to the stimulus signal, otherwise the result would barely sound like music. If a speaker is able to produce all the output levels needed for a pure sine wave, it should have no problem producing a slightly quieter sine wave right afterwards.
I think that when one talks about a speaker as being good at microdynamics, what you're actually hearing is some other factor that makes you notice the microdynamics more. For instance, if you listen to an acoustic guitar recording with a lot of high frequency content, you'll hear fingering and fret sounds that increase the percieved microdynamics of the track.
I'd argue that "this speaker produces better microdynamics than that speaker" might be a decent starting point when comparing two speakers, but in order to get something truly useful out of it you have to take it one step further, and try to determine what it is about that speaker that makes you notice the microdynamics more. That's just my two cents though.