The first is that Mitchell and Gerv probably were familiar enough with each other for her to speak to this. Gerv was very prominent inside the project, and Mitchell really did know everyone in the organization (at least in the triple digit or less days).
The second is that Gerv quite openly led with his religion, and this was incredibly controversial. It caused a lot of churn when Mozilla was working out community guidelines and that discussion was anything but private. I'm sure that led her to think that speaking to that publicly was essential.
The third is that Gerv was more or less impervious to criticism about his religion or his attitude of being open with it in all arenas, and probably wouldn't have blinked at any of her take. There was a lot of open conversation internally (and IIRC externally) about it. I suspect this led her to think that speaking as frankly as she might have to him or with the org internally was appropriate.
In total, I think she was wrong on a number of counts, the biggest being that the time after someone's death is about everyone else's reconciliation and they weren't coming from the same perspective Gerv would have. The optics were awful, and a combo of natural perception of position and her detached writing style makes it sound like a CEO crapping on the line from on high. The Mozilla "relationship" structure was much flatter than that, though, and with Gerv's community prominence and impact they were closer to peers than one might imagine.
Honestly, this shouldn't be the incident that colors her legacy. It was a pretty ill-advised blog post, but I don't think it's as malicious or even harmful as people make out, knowing the names involved. It's really hard to get across exactly how unshakably comfortable Gerv seemed to be with his faith and exactly how much that colored his relationship with the org.
I can't imagine how she could have acknowledged his impact without at least touching on some of that--even if I do think saying nothing at all would've been the winning move.