Here's the detailed transcript:
[46:26] Q: I have another question. In... Running with Scissors, was actually there something between Hekapoo and Marco? Because it looked like it potentially. (The audience says "Ooooohhh!")>"I don't know, Marc—er, Marco went a long time with being on his own."
[46:42] Q: So like, in the psychology of his character, I mean, he grew up, up through 30. And... he came back to Earth and he's like in 14, but I'm sure he hasn't... uhh, forgotten everything he learned in—in Mewni. So...>"Yeah, he's—he's, ya know, he's always been an old soul, I feel like, and I think that uhh... it just... he's an old soul, but still. But he has his memories, yeah."
I'm surprised, I would have expected her to flat-out deny it. Instead she says it's uncertain, but purposely reminds us that he "went a long time with being on his own" (16 years).
IMO it's practically confirmed, but she can't say it out loud.