Having used low antennas most of my radio career, I take most of this 'NVIS' stuff as a justification for a particular antenna or as an excuse.
When talking about antenna height you don't 'think' in feet or yards, you think in terms of wave length at the frequency in question. Then you can convert those wave length numbers to feet, etc.
Lower frequencies tend to 'bend' a bit when they hit the various atmospheric 'layers'. The higher frequencies (VHF/UHF) don't do that 'bending', they just go straight through those 'layers', and that's where that 'line of sight' comes in. (confused yet?)
Oh well, lots of reasons why you get various 'ranges' on a band, propagation being the biggy.
Paul