It's fun to speculate on these sort of white room match-ups, but I wouldn't really call it a problem. The demon lords (along with literally every character in every RPG) should be more than the sum of their stat blocks
The problem with looking purely at the start block of 5e monsters, is that they were created to simplify combat between players and to give them a chance to win, not display ALL of a monster's abilities.
While these are both true, there are two problems:
1) This isn't something that a lot of newer players know, and
2) Demogorgon being at a disadvantage (let alone not being superior) in a straight stat block match-up is a change from previous editions and existing lore.
So I understand why it bothers some people, even if it's surmountable with relative ease. Heck, it bothers me, even though I know I can easily adjust for it.
I swear, 5E has a grudge against all my favorite archfiends. Demogorgon is nerfed, and not only is Anthraxus not even detailed but they've changed "oinoloth" from his title into an average-powered species of fiend.
*shakes fist* WOOOOOOOOOTCCCCCC!!!