Levels in a videogame will differ from levels in a tabletop game.
That is not immersion breaking and the encounters are designed to grow in challenge over the cause of the game.
Just like in any other game.

Looking at familiar numbers being different is of no use. Lets focus on the big picture rather.

Easy for me to say maybe.
I am not a player of the "usual D&D".
But I must say I find the concept of certain beings bound to certain levels pretty uncreative, lazy and meaningless.

There is no reason a normal Waitress cannot kill any other Mindflayer if said squiddy does not see the femme fatale coming from behind with a Knife in her hand and she sinks the blade hilt-deep into its neck.
" B-but the challenge rating, th-the numbers, th-th... "
Dude they are just a wierd form of monster-mutation Humanoids and their armors looks like sh°°. grin

Their biggest defense are their psionic abilitys. Slip by that and they are WEAKLINGS.
Look how much they sweated when entering Avernus. Mindflayers too cannot escape the law of positioning. And their positioning was trash. grin

Even so, the Mindflayers are powerful in contrast to our starting characters.
So their glamour of might and power is well preserved I think.