Let me play devils advocate here...
Spino is bigger than I.rex. Arguably more powerful as well.
Spino had an easier time with T-rex than I.rex did.
Spino also showed some intelligence. Not to the level of I.rex but it should count for something. Mainly that it isnt just a dumb carnivore like T rex.
Spino survived the T-rex fight ,explosions, being burned and was pretty much unscathed. Last we see of him he just runs off fine.
Spoiler:
best i could do