Good question. I'd just chalk it up to AT deciding to reuse older panels instead of create ones where he'd have to demonstrate Future Goku's defeat of them in a different manner. The main issue with the reused artwork is the fact that he mentions Trunks right after, which could imply that Cell took the cells from Freeza and Cold while Trunks fought them.
I tend to just look at the other things Cell said. He was surprised that the Androids hadn't took Vegeta out yet, but the only thing he says about Goku is that him being alive is completely different from what he knew in his timeline. Cell would've probably said the Androids killed Goku if that were the case. This tells me the heart virus was what got Goku in Cell's timeline.
Judging by the way Cell showed surprise at seeing Vegeta and hearing about Goku, he probably assumed the events of his timeline were very similar to the main timeline. Well, except for the fact that Piccolo was merged with Kami.