Is Moon Knight teasing a secret third personality or a comics cameo?

polygon.com:

With its fourth episode, Moon Knight has really hit its stride as a series, but the show hasn’t run out of secrets to reveal or twists to tease. “The Tomb” had new characters, new locations, and a whole new way of looking at Marc Spector and Steven Grant’s messy relationship. And it revealed something else at the same time, by giving viewers the obvious hint that there is a third personality sneaking around the edges of the show.

[Ed. note: This piece contains spoilers for Moon Knight through episode 4, “The Tomb.”]

“The Tomb” concludes with Marc trapped in what appears to be his own mental space, in the form of a sterile, white mental hospital ripped straight from Marvel’s own Moon Knight comics. Just after Marc finds Steven inside a stone sarcophagus, and just before they both run screaming into the hippo-headed Egyptian goddess Taweret, they pass by another room with a sarcophagus in it.

This one’s a lot creepier, standing upright and shaking from side to side as if someone was trapped in it, and they understandably pass it by unmolested. It’s the series’ most obvious hint so far that there is a third personality sharing Marc and Steven’s body, but it’s not the first.

That came in Moon Knight’s second episode, “The Friendly Type,” when Steven experiences a blackout while battling some minions of Arthur Harrow’s and wakes up to find they have all been violently subdued — and Marc has no memory of killing the men either. As in “The Tomb,” Moon Knight moves swiftly on from that revelation without dwelling on it.

But for the savvy viewer, this could call into question every other one of Steven’s blackouts that Marc has not directly corroborated. Was it really Marc, who recently left his wife because he fears for her