In my understanding the only hard assumption in the grabby aliens hypothesis is that there is a chance of a civilization becoming "grabby" by which they mean it

In my understanding the only hard assumption in the grabby aliens hypothesis is that there is a chance of a civilization becoming "grabby" by which they mean it sprouts new civilizations in neighboring solar systems. Everything else is just necessary simplification. It even accounts for civilizations not becoming grabby. And the grabby hypothesis predicts we are quite early.