Of course 20 vs. 21 wouldn't make it "perfectly fine" for me. But I admit that the 21 does annoy me, meaningless though it might be. Maybe because it's meaningless -- they can't have made this NPC superhuman for gameplay purposes, because it barely affects gameplay. So then why is this dude theoretically the strongest humanoid on the planet? Isn't that a least a smidgen weird?
I think they build these guys using standard PC rules first, then convert them to a more simplified MM stat block. The guy has 19 HD, which means that he qualified for an Epic Boon, which lets you go over 20 in a stat. Whatever feat they picked was irrelevant for a fight, so it was removed, but the stat increase stayed.
I don't think its any more complicated than that. He's a level 19 Fighter that's been simplified. Military folks, such as guards, soldiers, generals, etc are Fighters. They figured 19 was a good level to make a challenge for a party on the verge of tier 3.
I suppose then a fair question would be what it says about the default world building assumptions if we have level 19 people running around for the party to challenge, but that's a completely different discussion than what's going on here.
That respec after a nap thing does a whole lot more than the "just makes it easier to CharOp" kinda thing that some folks are writing it off as and it's a big problem. When a PC can do something like be energy resistant and change the energy type on a rest it means that PC can never exist outside of a blind one naughty word where the player doesn't know and couldn't know anything about what they might encounter.
As far as I know, the only one who can respec resistance is the Fiend warlock, and that's been there since original 5e. It was originally playtested back in 2013 or so to be resistance to EVERYTHING, even better than totem barbarians. The playtest called it as waaay OP, so twas dialed down to this.
So, that's not really something new for "easier to Char Op" but a lesser version of allowing the Fiend Warlock to do their thorn-tank thingy.