In a presentation of this short (you can find the video on YouTube) he says he is conscious that cavemen and dinosaurs didn't live in the same period, what he was interested in is the contrast between the giant figure of the dinosaur and the small caveman (contrast wonderfully represented in a particular scene you will certainly notice). Also, he tells that this has nothing to do with Barney, it was pure coincidence (Barney & Friends first episode: 1992). Really nice short, considering this was made while he was a student.