Frankly, that is just a big pile of babble.
but “people” is defined [SIC] around the average person
There's no "definition" here. The closest to what you said that would make some sense would be "but "people" implies a generalisation around the average person", but it doesn't work in your argument because it does not contradict what BananaTrifleViolin said. Nor it justifies your assumption that
by saying “stupid” is not defined around average intelligence, you’re really criticizing the phrase “people are stupid”…
I genuinely think that you did not understand what the other poster said, so I'll repeat it under different words.
The comic has an implicit definition of stupidity as "lower than average intelligence" (see panel 2).
BananaTrifleViolin is highlighting that this is not the definition that people use for "stupid" when they say "people are stupid". And that leads to a fallacy called "straw man", where you misrepresent a position to beat it. Munroe (the cartoonist) is doing this, either by accident or on purpose. (It is not the first time he does this; his comic about free speech also shows the same irrationality.)