The point is that if grabby and gardener aliens exist, grabby aliens will dominate.
It's similar to the idea in evolution that we species that like to reproduce more than species that don't.
It's similar to the idea in evolution that we species that like to reproduce more than species that don't.
Perhaps grabby aliens, a known menace to life in the universe, are systematically exterminated when they are identified by other aliens in the universe.
There are really strong implicit assumptions at play here. This is a very interesting thought experiment, and it's nice to see novel thinking in this area. It's important to recognize however, that this is not "logical" or a "forgone conclusion" in any meaningful or systematic way.