The worst part of this comic is that philosophy bro is clearly not even very good at his field, since there's a much better Cartesian parallel to be made here (and I'm not even a philosopher).
"I think, therefore I am" is actually leaving out (imo) the most important part of Descartes's argument. He was trying to find literally anything that he could know without a doubt was true. The problem is, that's really hard, as our existence-troubled shopper has discovered. Descartes could doubt the existence of God, he could doubt the existence of goodness, of truth. All of these things might not actually exist. Descartes could even doubt his own existence.
In fact, literally the only thing Descartes could conclude without a doubt was true was the fact that he was doubting at all. So, since that's the only thing he could be sure of, that's what he built his argument for rationalism upon.
This perfectly mirrors the existential crisis the so-called philosopher comes upon, but instead of starting the shopper right where Descartes started, he instead just provides what must seem like almost a non sequitur in context, since if the man is currently doubting his existence, he can also doubt that he's thinking. What he cannot doubt, is that he is in fact doubting.
"I doubt. Therefore, I think. I think, therefore I am."