Yes that was the retcon explanation. The actual explanation is that they wanted to have Velociraptors in the movie and weren't really bothered about the fact that they aren't actually that big, there are species of raptor that are that large, but they didn't want to use their names because they were less well-known. Velociraptor was one of the few dinosaurs people knew. T-Rex didn't become famous until after Jurassic Park.