"It isn't here, as far as we know. I've promised not to do it on purpose."

Yes, someone had to extract that promise from him.

"Parallel worlds, presumably. Because if Gen and I -he's someone from my world- were from the same place, when I vanished which is further back in time from him, he would've noticed." Senku commented. "He didn't, as far as he knew I was with him the whole time and I definitely would've told him if I came back before him coming here."