It's a fairly funny joke here, not the usual source of humor in this one. Most comics (including Peanuts up to this point), it would just be enough to stretch the first two panels here over four.
Consider, for a moment, the comic strip Nancy. Nancy is, itself, a kind of classic, an endless elaboration upon a basic set of jokes. And yet, it cannot really be said to have evolved over time. Ernie Bushmuller was a craftsman. A really good one actually; few comic strips could have maintained the level of competence he provided for Nancy over that period. That is a good word for what Nancy is: competent.
Schulz, we see here, was not interested in mere competence. We can see here that he wasn't interested in applying a formula over and over again forever, that he was engaged with his work and responding to it in an iterative manner. In this strip, he comments upon a kind of joke that just a year earlier he would have made without second thoughts. This is why Bushmuller was a craftsman, but Schulz was an artist.
