...Somehow I kind of seriously doubt this. A SECRET HELL LEVEL!!!!11oneone?? You mean there was something worse than The Acrobatic Act From Hell(tm)?
It's possible it could be a reference to what everyone seems to speculate was Milla's original level design, but that was as far as I can tell completely cut save for the crazy sound files you can find with Psychonauts Explorer. And I've never even heard these myself, considering I have the PS2 version and all.
But right, 'tis probably just a random uninformed reference to Milla's possible original level design of doom.
“Raz,” Oleander groaned. “How many times do I have to tell you? You don't have to sneak in here past security every time you come anymore! You can just come in through the gate! You know, like everyone else?”
“Oh, I know. I just like this way better.”
, chapter two, by Digitaldreamer.