Just my personal take, I think this is a really well-written incident postmortem. It's specific, extensive, candid, and dare I say, entertaining?
Many incident reports are fully lacking in any meaningful detail, or wholly unapologetic. I actually enjoyed learning tidbits about the author, in particular their mention of https://how.complexsystems.fail/.
Reading this boosted my confidence in Slack's teams, which should ultimately be the objective of a release like this. It's not pure PR nor a gruff legally-obligated disclosure.
It helps that I wasn't really affected by this incident.