Possibly not a great choice of words given that the article mentions that actual post-mortems are currently being performed.

What word do you suggest he use? If anything, I feel like 'postmortem' most commonly implies an after-action report - with the specific word 'autopsy' being used to describe, well, autopsies - at least in US english.

This is presumably a US English v English thing then, as post-mortem wouldn't be the term used here usually, and certainly not if someone had died (unless if was being performed on them). To someone from the US it would perhaps seem a little distasteful to describe the investigation as an 'autopsy' - hence my comment.

Oxford "British and World English" dictionary, post-mortem: "An analysis or discussion of an event held soon after it has occurred, especially in order to determine why it was a failure." https://en.oxforddictionaries.com/definition/post-mortem. Maybe not common in the UK, but I'd say it's perfectly correct word usage.

I grew up in USA Midwest, though I have been accused of a "southern" accent. "Postmortem" is completely idiomatic in this situation, but if you'd care to suggest a better term we'd all learn something.