Er...I'm pretty sure AsstEvHmnRcrsDrctr was being sarcastic.
The statement about "No." not being a sentence was in quotes, as though someone else was saying it?
Questioning why a comment like that wasn't here, pointing out that almost every other time SA makes a slip-up with grammar, commenters on here are jumping all over it?
Finishing up saying he misses it, when no one in their right mind would?
Pretty sure that all implies sarcasm.
In other words? Settle down.
AsstEvlHmnRcrsDrctr : get over it. IF you're so concerned with grammar, be an editor in a publishing house. I'm a grammar fanatic, but sadly (I guess) the grammar of comic strips isn't really that interesting to analyze.
But you asked for it... In this case "No." can be a sentence. Because the implied meaning in conversation is "No, I don't need help with my project". It's just truncated for the colloquial purposes of not repeating redundant information when it can be derived from context. Considering this is a comic strip illustrating conversation between two people, and not an essay, that's perfectly fine.