None of it was because the scientists thought that dinosaurs were amphibians. In the lore of the books, DNA from reptiles, avians, and amphibians were used to fill in gaps in the dino DNA.

The book explains that the DNA used to fill those gap was chosen specifically for certain traits that would make for more attractive or durable creatures for the theme park, highlighting how what was being done "in the name of science" actually had nothing to do with any real scientific progress or discovery.