I don't believe that Shadow People are necessarily anything to do with Humans. In the astral plane there could be a whole new spectrum of species evolving under different circumstances.
Lots of Astral Travellers have encountered Astral entities (including self).

The Humanoid Shadow person could be just as intelligent as we Humans, and neither good nor bad. It just needs to feed as any other creature dependant on energy.
We consume foodstuff and convert it into vital energy. SP's absorb energy from us (and possibly others).
The fact (IMO fact!lol) that it absorbs energy maybe why it appears to be dark? It emits or reflects nothing. Human spirits emit light (in my experience).
That maybe something to to with the receiver and transmitter wavelengths of both?

The incubi maybe a species from the astral plane like the Shadow people. There could be as many species there as there are here.
Who knows?

(By 'there' I mean 'here in a different wavelength'.)