I'm disappointed by the line that NPCs gain class levels using the same rules rules as in the PH. I was hoping for some more simplified approach for DMs who don't have tons of time (especially since we clearly won't have a digital tool any time soon). I like most of 5E, but there are a lot of little things that make the DM's job harder than they need to be.
RAW, I can understand your disappointment.
However, as a DM, my NPCs only get as much detail as they need. either as world flavor or to last 5-10 rounds of combat.
The arch-master dwarf craftsman? 1st level npc with 20th level proficiency bonus for crafting only.
BBEG? Maybe full stats, key proficiencies (like NPCs in the MM) and whatever unique abilities he has. Might be class based (most of the time), might not.