That's what I mean. If I have to alter the stats of a monster to work in a normal way, then it "doesn't work".
He didn't say to alter the stats (at least not in the quoted portion). He said to "beef up a boss monster's defenses." That does not have to have anything to do with stats. Just offhand, that is: traps, lair design that prevents focus fire from PCs, mooks, mirrors/illusions, protective items, or decoys. None of which touch the base stats (except perhaps protective items, which if you can't add a modifier for a magic item, you shouldn't be running Legendary monsters anyway...). Defenses are far more than individual power. But the problem is that it seems like a lot of DMs see "Legendary" on a stat block and therefore think the monster is an idiot who will only engage in a face-to-face "fair fight". If you (a person, not you as an individual. No ad hominem) can't divide, disperse, and deceive a PC with RAW Legendary Creatures, then yeah... it's your fault, not the game's.