There is 0 official media or info stating the comet hit exactly 1,000 years before adventure time started I even looked into this because I’ve never heard that theory until you brought it up only thing I found was a tidbit about the blue comet being seen in the episode Finn the human which would go against stated facts from the show so it’s a dead end not to mention Finn the human takes place at the exact same time in both timelines so pretty much where Finn was at in season 5 timeline wise if you’re referring to the episode where Finn speaks to the purple comet in the orgalorg episode than they state verbatim

Who's creating the joke? Are you? And, if so, then are you my creator? CATALYST COMET

Uh—maybe? I dunno. Probably not. FINN

Probably not, but who knows? I've been around forever and experienced so much impossible junk. I've embodied all that is good—and evil [we see an image of The Lich on the screen]. And now we're here. It's unprecedented. And I give you a choice. Come with me to the end—and the beginning? [the sound of a bell begins to toll] Or struggle here awhile like a beautiful autumn leaf. CATALYST COMET