Well, everything you know about featherless dinosaurs is
also based on the guesswork of science. Everything we know about all prehistoric creatures amounts to guesswork.
Saurian is grounded in the "soft dinosaur" movement, which is explained very well in
this video. The Soft Dinosaur movement encourages speculative behavior as a reaction to earlier theories which only supposed truth based on direct fossil evidence, despite the fact that many creatures we can observe in our world today had bizarre features or unusual behaviors that cannot be observed from even the most complete skeleton. The Soft Dinosaur movement is
technically more correct in its theories than most creatures shown in the JP universe, because the Soft Dinosaur movement at least has skeletal reconstructions that are far more accurate.
I'd actually like to point out that the Metriacanthosaurus is
breaking from JP lore in one very significant way: its hands are supinated, not pronated (held in front of the body like a rabbit) like most JP theropods (Rex, raptors, Spinosaurus). We know that very few theropods could actually pronate their hands. Basically, this Metriacanthosaurus is 100% accurate to our current (but very limited) understanding of the animal.
Off topic, but if your nostalgia for Jurassic Park is ruined by the possible appearance of feathers on dinosaurs in a video game... just go watch Jurassic Park then?