It does move both slowly and rapidly. The currents of air layers in the atmosphere vary over time making it appear to dance around. Usually more movement asit rises low in the sky and less so as it gets further overhead.
When it zips from horizon to horizon in less than three seconds, get back to me. Even the wave forms due to a heated highway can cause a ‘dancing’ effect, but the analogy is to an object moving around not just shimmering, so the author’s assertion is balderdash meant to divert from reality, to manipulate folks’ reasoning.