So on what basis is someone's performance reviewed, if such performance is omitted?

The entire point of blameless postmortems is acknowledging that the mere existence of an outage does not inherently reflect on the performance of the people involved. This allows you to instead focus on building resilient systems that avoid the possibility of accidental outages in the first place.

I know. That's not what I'm asking about, if you might read my question.

I'll play devil's advocate here and say that sometimes these incidents deserve praise because they uncovered an issue that was otherwise unknown previously. Also if the incident had a large negative impact then it shows to leadership how critical normal operation of that service is. Even if you were the cause of the issue, the fact that you fixed it and kept the critical service operating the rest of the time, is worth something good.

I know; that's not what I'm asking about. I'm talking about a different issue.

Mistakes happen, and a culture that insists too hard that "mistakes shouldn't happen, and so we can't be seen making mistakes" is harmful toward engineering.

How should their performance be evaluated, if not by the rote number of mistakes that can be pinned onto the person, and their combined impact? (Was that the question?)