No offense, but being a professor (or having a web site) doesn't mean you're always right.
Of course, you could say that Merriam Webster doesn't have the market cornered on accepted spellings either- but the language does in fact fluctuate, which is what makes it enjoyable, in my opinion (although I'm not a big fan of "ain't").
Incidentally, there aren't any spelling errors in this strip, unless you're debating the spelling of "sewerside," which he cleverly concocted, and therefore gets license to spell however he chooses. One thing I've noticed Scott is adept with, and that's basic English. How refreshing.