Would anything limit an "invasive species" T. rex introduction now besides equilibration with available prey or intervention by humans?
Lots of things could impact a reintroduced invasive species.

Disease/sickness: although no close relatives are present that is not a guarantee,  many diseases jump species. Parasites likewise could cause problems, especially newer parasites the dino may not have a good defense for. Even modern Fungus could pose a problem for eggs.

Predators: While an adult t-rex is predator proof eggs and newborns are not. Rats and other small mammals as well as many birds have been the demise of many ground nesting birds.

Poisons: there are plenty of animals and plants that are poisonous enough kill a t-rex that the rex may not recognize as dangerous. Remember there are a lot of things around a t-rex did not evolve with. This is more dependent on location, a t-rex dropped in Australia might kill itself with its first meal.

Human stuff: I am not talking about humans killing them. I am thinking of the things humans make killing them. things like walking into power lines, eating weird plastic human garbage, car accidents ( a car moving at speed could easily cripple a rex), even something as simple as getting hit by trains