This is an emerging theme of Peanuts strips, where one character reads something in a book and reacts to it. Often these reactions are solitary, just a way to present a statement for a character react against.
Usually these statements are meant to be accepted uncritically. Not in this case however, here Snoopy's reactions directly disprove the book's declaration, which makes the strip into a commentary upon it.
As for whether dogs really can't reason, well, I doubt that severely. I've known a couple of very smart dogs in my time, one really quite freakishly so.
Charlie Brown's personality is still fairly mischievous at this point. He doesn't do this much more often. It seems like all the mischief in his soul disappears as it increases in the other characters, particularly Lucy. So maybe there's some kind of spirit transference going on here.
Scribble of ire!
