why humans absolutely dominate every animal on earth - our intelligence.

We like to think that about ourselves, but in my opinion, that’s not the whole truth. After all, there are other intelligent life forms as well (though it’s certainly debatable whether they’re just as intelligent, “differently” intelligent, or simply less intelligent). The key reason why we, in combination with our intelligence, dominate the planet is, in my opinion, cooperation within social groups beyond the (extended) family. Religions, states, laws… all mental constructs we use to ensure that we can cooperate within massive groups. As far as I know, no one else does this.

In our scenario here, this crucial component is missing: While a human is (quite likely) more intelligent than the T-Rex, he is, after all, just one individual, and his true strength lies in the community. If we were to place 50 people together in this setting, I believe there would be significantly less doubt about the outcome.