Disregarding the weird tone, the letter makes some interesting points.

  None of us is the best person out there.

This whole notion of someone out there being the best programmer (or person, or whatever) is fallacious. While there are certainly many degrees between "awesome" and "failing", it's important to recognize that either you're indeed awesome at what you're doing right now or it's time to move onto something else. Of course, the definition of "awesome" is completely subjective so you have a lot of room for introspection there.

  That’s irrelevant! Because we are best for each other.

This is an important sentiment, certainly one of the most crucial ones in a co-founder relationship.

I have seen what it looks like when it's not working out: we were friends before the startup, we liked to talk about crazy ideas all day. But a while into the startup, all we talked about was money and disappointment. This created an atmosphere of misery and failure, long before the actual failure of the company. If I were to do a startup again today, I would look for this kind of synergy the author is describing. It's important. It's not enough to just be a bunch of dudes hacking away at stuff.