So the game is deliberately rigged for them to win then?
That's a different story.
Correct. Demogorgon has heavily reduced stats and no minions based on the story (you collect components for a powerful spell to summon all the demon lords to 1 spot where they'll fight one another because that's what they do: destroy and consume). So he also wouldn't have lair actions or regional effects.
After that, like pretty much all demon lords and demons, his spells are a joke, his attacks while nasty, are limited in #. A prepared party at 12+ levels is going to have magic that'll help make him miss. After that, everyone is going to have magic weapons and probably know by now what spells not to use. With all that in mind, many PC weapon characters can easily do 100+ damage in a round by now.
He's nothing like his older forms, and perhaps that's intentional for the adventure to fight a watered-down version so you can put that on your character lifetime achievement board, but I didn't much care for any world (as I said in my last post) wherein in about 1 year, some mortals become strong enough to take out a demon lord that other mortals, devils, and demons have failed to topple over thousands of years. You used to have to be 20th level, decked out in artifacts, to stand a chance.
So, I buffed all of them up, and had an epic session by letting my PCs control demon lords in a big melee. Whoever survived the Royal Rumble was the target. A buffed Yeenoghu with around 100hps left was still scary, and more so if had I given him some version of Rakshasa magic resistance so he wasn't so damn vulnerable to
magic missiles.