Okay here’s the thing.
"They" didn’t "announce" that Marceline and PB were a thing. The voice actress for Marceline, Olivia Olson, who is a delightful person, said at a public event that Pendleton Ward, who created Adventure Time, once told her that Marceline and PB had been a thing, but that they couldn’t show it on TV because in some countries homosexuality is illegal. She later tweeted not to take everything she says at events seriously, then deleted it.
Do I doubt that Pen said that? No, not at all! I’m sure that’s how it is in his mind. The fact is, though, that it isn’t on the show, and it isn’t in the books. It can be the unacknowledged backstory, but unless it’s shown…
I want this as much as you want this, maybe more, and about as much as I don’t want to put my writing Adventure Time books in jeopardy. I’d be thrilled if they said it on the show or let me write it in the books. I know that they were a thing. I know the writers know that they were a thing. What it matters is making it visible, and it isn’t, and while I understand why that is for a million different (bummer) reasons, it’s not “official.”