The issue is pretty simple, really. The loudspeaker's baffle forms a half-space launch point at midrange frequencies up, but at low frequencies, baffle dimensions are much smaller than the wavelengths generated. So the baffle is insignificantly small at those frequencies and below. But then, the walls become significant at some frequency too. Usually, if the speakers are near the walls, then there is a frequency region between where the baffle stops acting as a half-space lunch point and where the walls start. But the point is that baffle-step compensation has to do with compensating for increased midrange due to the increase in DI from transition from free-space to half-space.