I think people are forgetting that a severance package is not a legal requirement, and that this one is actually pretty generous. "I fucked up, here's what I can offer" is more or less what this letter is saying, but it's just not drawing the connection between the two parts in a way that makes sense to people.

People can disagree about whether the package is satisfactory, but that's completely beside the point. The severance package is the make good Zuckerberg is offering: it is an expense his company is paying when they do not have to.

It is the implied answer to the question "Oh yeah? How is he taking responsibility?" Argue about whether it's good enough, but it is an answer.