He says in that scene that they created an alternate 1985 and can’t go back to the original timeline without undoing the very creation of the alternate timeline.
Why can’t you, if both timelines exist?
Because they have a time machine, not a universe-hopping machine. They’re stuck in a particular timeline, and if they create a divergent one, they get stuck in the new one until they can undo what caused the creation of the new one. If they had a portal gun like Rick from Rick and Morty, they could just say “Ah, fuck it, too much work” and hop back to the original timeline.
Think of it like train tracks. When they create a divergence, they create a switch on the tracks to a new railway, but they don’t have the tools to simply move the switch back to the original railway to get back to it. They have to dismantle the switch altogether, which means going back slightly further than the creation of the switch and preventing its creation.
Another fun thing to think about: they’re not even trying to go back to the original timeline, since Marty created a new one in BTTF1 when he went back to 1955 and got hit by the car instead of his dad. The timeline created in BTTF2 is actually the third timeline. They’re trying to get back to the second timeline. Which raises the ethical question of, what about the original timeline? Do that version of Marty’s parents think their son went missing/was kidnapped by Lybian nationalists? And what happened to the Marty from timeline 2, the one who remembers growing up with his “fixed” parents? When he went back, did he change something else and wind up coming back to a fourth timeline? Did he get stuck in 1955 while timeline 1 Marty got to go back to the future? Is there a timeline where two, or three, or four Martys show up on 1955 Doc’s doorstep?
Time travel is confusing and tough to write, man.