It doesn't have to be funny, but it does have to be distinguishable from the thing its satirizing. You could run this in an a blatantly xenophobic newsletter and no-one reading it would bat an eye
Let's say you're just switching channels on your tv. You come across Louis CK, who you never heard of, and he says 'I can't stand black people!' (literal quote from one of his bits). You turn off your tv in disgust, and start picketing him, because hell, what possible context could there be for that? The guy must be racist!
Replace Louis CK with Colbert. Or anyone. It's fine if you didn't get the context immediately. It's a random page, in a (famously so) leftleaning satirical magazine, that's supposed to accompany one or more texts about the subject. Your initial reaction is shock - which is exactly what they're going for - and because of a lack of context you don't get it. People here explain it. You understand the context. Now you're mad because...?
It's not meant to be Garfield. They're not trying to give you an easy laugh. They're aiming at a slightly higher level of social/political consciousness. Shouldn't they be allowed to do that in their own magazine? Yes, if some insane rightwing website made a joke about how we should just let muslims drown because they're not christians, it's fine to be insulted about that. But that's not what this is, so I really don't understand what you're trying to say.
Being offended is fine. Never being offended by anything isn't a human right.