If it's vision is based on movement...

Whatchu talkin' bout, Willis? 🤔

And, for that matter, how the hell was Grant so certain that his theory was correct? Lucky for him he was right, but I struggle to see how a palaeontologist had surmised this from studying fossils.

He means in relation to other living creatures. The T-rex was most certainly aware of solid, non-living objects, but it hunts by sight, watching for prey to move so it can catch it.