It made me laugh, but it's too broad of a generalization. Some news aggregators really add value. They drive traffic and stimulate interesting discussion. They make it easier to get opposing viewpoints on the same topic. I wonder if Scott Adams has a chip on his shoulder because of the effect aggregators have on old media, his bread-and-butter. I find it hard to imagine XKCD labeling all news aggregators as parasites…
I think this was really more a joke about Rupert Murdoch than his personal view, especially since Murdoch's 'parasites' comment has been discussed a lot recently.
Edit: adding link to show a reference http://www.mediaite.com/online/google-rupert-murdoch-decries...