Since the race was on public roads and not at the track, it doesn't matter if Julia and the buyer say they saw Dino at the track - they can't say for sure that he WAS at the race, and Dino had witnesses to say he wasn't.
The most they could definitively say is that Dino agreed to race. And even if they did say as much to the police, Dino just has to say "I didn't show up, here are my witnesses to prove it" and unless the police have enough information to go digging into those false alibis (presumably they had no reason to distrust the witnesses) this would be accepted, and Dino cleared of any involvement. Tobey was absolutely there and not denying the fact, plus he was distraught over the death of Pete so probably in shock at the time.
I always thought the fact that the Koenigsegg Ageras belonged to Dino's uncle (? I'm going from memory here, might have been some other relative) should have made the police look more closely at him, but clearly they didn't.