Earlier quoted context omitted.
I thought it was a fairly common turn of phrase! https://blog.codinghorror.com/the-project-postmortem/
A postmortem is for reflecting on something that went so wrong you had to kill it, and you want to objectively describe what led to that, and what lessons can be learnt from the failure. A retrospective is just the reflecting part, without implying you believe it is going to fail and you're looking for some schadenfreude ;)
You’re taking it very literally here, it does not involve anything being dead or killed.
We’ve used this in the software, games, film and construction industries for decades at this point.
Substitute Mortem for Ship and that’s how people use it.