Not quite.
Let us start extreme. Gods and Demon Princes. Completely NPCs, and they use rules that are not available to the PCs.
Is that cheating? No, Player's are not meant to have the sort of far reaching cosmic power that those ancient entities are supposed to have access to.
Bodaks are worshippers of Orcus that have turned themselves into monsters using a dark ritual. Same with Liches (the dark ritual part at least) these are not PC options. Turning yourself into a Bodak would immediately place you into the realm of NPCs. Again, is this cheating? I don't think so.
Hags have powers that players don't have, Dragons, Aboleths, ect.
I think, given these examples, you might be kinda missing my point.
None of the creatures you reference are generally considered to be playable as PCs, and therefore one doesn't have to worry about any comparison. I don't need to concern myself at all with giving my NPC Demons and Bodaks and Aboleths the same abilities as PC Demons and Bodaks and Aboleths as there will never be such a thing. (exception: if a PC temporarily polymorphs into a usually non-playable creature it goes the other way: the PC then has to conform to the monster write-up unless the effect that generates the polymorph specifically overwrites that)
When I talk of NPCs here I'm specifically referring to those of a normally-PC-playable race or species.
Now, I agree, if an enemy wizard uses a spell, and the players find that wizard's spellbook, the players should have the potential to learn that spell. It is possible they could follow a similiar path to power. But, to build NPCs with no regard to player rules meant to enforce metagame balance on the system, is not cheating.
Whether or not it's cheating (which is probably an overstatement anyway) depends on one's view of setting fidelity and consistency, I suppose.
I like to think of it as in a given setting all Humans* operate within the same mechanical parameters e.g. 3-18 attributes on a more-or-less bell curve distribution, can't see well in the dark, normal lifespan maxing out at 80-100 years, stuff like that. Further, any Human* has the ability to gain xp and advance in levels, though not all do so for a variety of reasons; and we happen to play some that do.
* - or Elf, Dwarf, Hobbit, Gnome, Part-Elf or Part-Orc; changing the examples to suit the race.
Thus, PCs are much less special from everyone else that some people would have them. Yes they use a more generous method of stat generation, but all in all that's about it for their "cut above" status; anything else comes from what they do in play.