I have Asperger's syndrome and I'm not offended by what he said (maybe because he wasn't trying to insult us, or SpongeBob, at all).
Also, back when I was in elementary/middle school, I really didn't know how to pick up on certain social cues (eye contact and tone being a couple of them). One of my fondest(?) memories of the first grade is spraying my teacher with saliva to show her how the people in Rock Bottom talked, which was quite rude and definitely not something I'd do now. I'm sure I've taken some things literally as well during those times, and I'm still inclined to take song lyrics literally, even if I do become aware of the metaphor later on. Obviously, these problems aren't as bad now (either because of the social skills classes I took in middle school or because I just plain grew up), but they CAN happen to someone with Asperger's. And actually, I can relate to the times when SpongeBob doesn't understand sarcasm because there have been times, even today, when I've had trouble understanding people's sarcasm, which is considered a symptom of this disorder.
Despite that, I do agree that SpongeBob does not have Asperger's syndrome because he lacks other symptoms. SpongeBob isn't the type of person to daydream much, which often happens in my case, doesn't create strange, near-involuntary movements known as "stimming," has interests (jellyfishing, Mermaidman & Barnacleboy, the Krusty Krab) but doesn't put too much of a focus on either one and seems equally knowledgeable about each subject, and doesn't speak as formally or use as many big words as l do most of the time. Also, SpongeBob is anything BUT shy and socially awkward, which is what most Aspies usually are. Then again, I'm just relating to this through my own experience with the disorder, and I'm sure everyone's experience isn't always the same.
And, most importantly, it's a CARTOON. SpongeBob's supposed to be an innocent, naive man child (sadly, most people think Aspies are like that last part as well, which I DO take offense to) and doesn't have any mental disorders because he was never designed to have any. It's just how the character is and I wouldn't have it any other way.