With the "grabby aliens" there is only the assumption that a portion of civilizations become grabby. We don't have good reasons to believe that portion to be zero. But even a very small chance leads to all the other predictions, like that we are early and so on.
It's also not so much about active contact than that you could observe technological signatures from a certain distance.
Our definition of "technological signatures" is an ant colony's pheromones. Because we don't have a solution to quantum gravity we cannot rule out the possibility that other civilizations can control space/time without emitting photons.
Our civilization is already putting out photons into the neighborhood. Hard to imagine other civilizations being able to master that technology in a blink of an eye when it's not even possible with all the physics we know.
It's also not so much about active contact than that you could observe technological signatures from a certain distance.