It sounds dumb and overly simplistic, kind of their version of the Prime Directive but it could be that simple. Any alien civilization that's hostile could have easily attacked us by now with little risk.. the few things we have that could do damage to something in orbit might not be nearly enough to take out a dedicated invasion fleet. Heck, them taking out our satellites would already cripple us because we've become so dependent on them.
So if there was contact or if aliens exist and have discovered us i assume they are either benevolent or simply don't care about us.
I don't know about your previous arguments being simplistic but this one certainly is. Technology is not humanity's strength and never was. For the sake of argument, an alien passing this planet would a) find it endlessly fascinating and b) unbelievably dangerous because of the clever and inventive indigenous alpha species.
Also, something advanced enough to make it here and look at Earth is highly unlikely to be interested in blowing all life to oblivion just for kicks. Just because science fiction necessarily makes alien visitors adversaries doesn't make it any more sensible in a real life scenario.