They aren't going to make them as PCs for a module, because the method they default to is the MM version. Their personal default does not negate the other two methods or render those two methods not RAW. There is no evidence in 5e that negates the fact that you 1) can by RAW build NPCs with PC rules, and 2) NPCs are by RAW monsters.
Your evidence isn't.
So, yuppers, you can build NPC's using the PC rules. Sort of. Again, since you were incapable of telling me what the CR of a 7th level monk was, I'm not really sure how, exactly, you build an NPC using the PC rules, but, let's move on from that.
The fact is, while NPC's can be built using the PC rules, they AREN'T. An Archmage isn't built using PC rules. Like, at all. A Gladiator is not a fighter subclass. It does not, in any way, use any of the class abilities of a fighter. None of the NPC's ANYWHERE in any published D&D material are built using the PC rules. But, sure, you can build NPC's using the PC rules.
Note, while you could build an NPC fighter using the PC rules, you absolutely CANNOT build a gladiator doing the same. It just doesn't work. What class is a Kuo-toa? After all, it's humanoid. So, why does it have 4 HD? Whereas a Kuo-toa bandit (since bandits can be any race and Kuo-Toa are humanoid) only has 2 HD. How does that work? A guard has 2d8 HP. What class is that? So on and so forth
So, yes, you are technically correct. It is RAW to use the PC rules to create an NPC. However, since it is never actually done, and, most of the time it's actually impossible to do and get the correct results, it's not really all that correct at all.