New interview with Kirkman in Gimple.
After reading it, the more I now firmly believe that they're not cannibals, precisely because that is what virtually everybody assumes they are. They seem to be trying to mislead the audience. As Gimple says, there's nothing there that explicitly says that they're cannibals if you haven't read the comics and don't know what is "supposed" to happen next. And if you read the comics, wouldn't you rather be surprised?