I think what they mean is "let's do this like a blameless postmortem" not "let's pretend that no one has ever done anything wrong."

If we can't be like, "hey the current state of affairs can be traced back to X, Y, Z" without the implication that whoever did X, Y, Z is literally Satan then it's no wonder we never get anywhere.

A blameless post mortem that blames someone and doesn’t identify what actually went wrong?

Look, it's a two sentence comment, you're reading the blame into it. Saying that Mark pushed the change that caused the outage isn't blaming Mark.

Saying that Trump (the original author, doing his best) thought that looser fiscal policy was a good idea and that Biden (the current maintainer) has to be the one to course correct due to unforeseen consequences isn't blame unless you take it as such.