I think they're actually jilted by Microsoft kicking the tires, then deciding not to buy them. By expanding Teams and integrating it into Office... well, when I work in that way the experience really is way, way, way, way (did I say way) better. Slack doesn't fit in nearly the same way, within that ecosystem.

At the same time, I think Slack has an exit problem. As you've said, they're selling chat. They have to hope to be acquired - but by whom? Google already has 12 platforms. Facebook has 2, and famously doesn't compete on this part of the business side. Similarly, many of my friends companies have left Slack for things like RocketChat, Chanty, or Mattermost.