Arguement seems to be based on two assumptions:

1. Space aliens would come to our solar system and do stuff we would notice.

2. The first bunch of aliens to colonize the galaxy would do nothing to prevent other aliens from coming to earth and doing stuff that we'd notice.

Since we know of only one advanced technology using species, ourselves, we can conclude nothing from our sample of one. We have no reason to conclude that aliens would act as we think we would act ourselves. In fact, when you look at history you can see our own attitudes have change dramatically over very short periods of time. One hundred years ago many supposedly civilized people had no problem with the idea of exterminating humans that belonged to what they called "lesser races." Nowadays, fortunately most of us find the idea abhorrent. A hundred years ago people wrote about the thrill of killing the last member of a species and making it extinct. Now we frown on that sort of behaviour. People used to look forward to the day when all wilderness would be replaced by farms and cities. White people used to be considered athletically superior to black people. Basketball used to be a Jewish sport. Since we can't seem to get our own motivations straight for more than a generation, I don't see how we can conclude much about alien motivations.