I'm not so sure. All sorts of things appear on flags that aren't to do with nationalism. You can put a smiley face on a flag if you wanted to, and hoist it above your outhouse. You can put a skull and cross bones on a flag and put it over your DJ mixing desk.

There probably is intelligent life out there. If we meet them one day, and they have a flag representing their planet. "Hi Earthlings, you don't have a flag?"...awkward.