You may say whatever you want about what would happen in a real Spino and T-Rex duel but in Jurassic Park the dinosaur's DNA was different than it would have been in reality because of the many shortcuts InGen took to make the dinosaurs. So like the JP Velociraptor was larger than in reality the JP Spino was stronger and larger than in reality. So in this case the T-Rex wouldn't have ended the fight when it bit the Spino's neck because of the DNA inaccuracies when cloning.
So basically what I am trying to say is that the story of the film is fine and isn't inaccurate to real life because in the story itself the dinosaurs were slightly inaccurate. And when I say "isn't inaccurate to real life" I mean that if we were to clone the dinosaurs and in the process we alter their DNA the same thing could have happened.
So if you say that the T-Rex should have won it would basically be the same as saying that the Raptors are too big or that the T-Rex eyesight wasn't motion sensitive or that the Diliphosaurs couldn't spit and stuff like that.