I've admittedly put in very little effort to obtain postmortems from Fastmail. But I also posit that I shouldn't have to. Postmortems should be made readily accessible, just as the outage was. I reserve my detective skills for figuring out my own service's outages.
It seems like your primary goal is to call them out, rather than to get an explanation, and so I don't really have any advice to offer in that regard.
These priorities and expectations aren’t universal, but they’re common enough and perfectly reasonable.
Also, since the parent didn’t mention writing to support, your response comes across as condescending and passive-aggressive. I’m not sure it was intended that way, but it reads as though you fully expected that the parent never wrote to support in the first place. If that tone was intentional, HN really isn’t the place for that sort of attitude. Simply suggest that they write to support—no need to be condescending about it.