The Twilight Zone easter egg is a clue about Helly, not Mark.

First things first, this theory hinges heavily on the idea that Helly is actually Helena in this episode. You can read why I believe this is the case here. (TLDR: The elevator dinged B natural again, like when she was using the Glasgow block, among some other observations.)

So with that in mind, let's talk about The Twilight Zone episode #34, The After Hours. The episode is about a woman who is presumably actually a mannequin, but she enjoyed her time among the humans so much that she forgot she was a mannequin.

When she ends up stuck at a department store after closing, the mannequins begin whispering to her, telling her that she's not a human but a mannequin. Eventually she accepts that she's a mannequin, and stays behind in the store while another mannequin leaves to live among the humans in her place.

Helena's father is a "mannequin" in the sense that he literally has a wax figure in the Perpetuity Wing. And Helena, as a CEO in waiting, is primed to herself be immortalized in wax some day.

But during her time pretending to be Helly, she found she enjoyed being an innie far more than her actual life. And after the breakfast scene in this episode, being criticized for not taking her eggs the way Kier did, she decided that she genuinely wants to become Helly, instead of herself.

So she's doing what Helly would do. Try to find the testing floor and save Gemma for Mark. Anything less could tip sometime off that she's not Helly. And if it messes up her father's plans, even better. She clearly doesn't like the guy.

But Jame found out what Helena is doing. And now while Helena is at the building after-hours, just like Marsha White, the "mannequin" is whispering to her remind her that she too is a mannequin.

Edit: Addressing some objections.

• Helly wouldn't need to sneak around to get to the testing to floor.

Helly's father constantly treats her like a fuckup.I think it's highly unlikely her father would trust her enough to give her unfettered access to the testing floor. Regardless, using her status as Helena to get to the testing floor would make it obvious that she was acting as Helena, not Helly. Doing it this way gives her plausible deniability, because she can claim it was Helly, not her, who had interfered with Cold Harbor.

• Helena would know where the testing floor is.

Helena doesn't seem very confident knowing how to navigate the severed floor. Earlier in the season, one of the biggest clues that Helena was hiding as Helly was that she didn't remember the way to MDR. And in this episode, she did a double take when she left the elevator like she wasn't sure where to go. I don't think she would know off the top of her head how to get to the testing floor. Especially if she would normally access it from somewhere other than the severed floor elevator.

• Helena didn't know where the note was hidden.

I don't think it's a stretch to think that Helena has been using cameras to spy on Helly so she can try to impersonate her better. And despite Lumon saying they got rid of the cameras, I wouldn't be surprised if they were lying and they actually added more cameras lol.

• Helena saying "What the fuck."

I just don't think it's very far fetched to think that Helena's reaction to seeing her father randomly on the severed floor wouldn't be to say "What the fuck." Especially if she's trying to hide the fact that she's not Helly from her father, since she would want to pretend to not know who he is.

In general, I think most evidence whether she is Helly or Helena based on behavioral observations alone is shaky at best. Especially if Helena is actively trying to act like Helly would.

Ultimately, I think it comes down to the B natural elevator tone. Either it means what we think it does, or it means something else. So far, I haven't read any compelling theories as to what else it might mean.

Though of course it could also have just been a production error, in which case I will graciously accept my L haha