Without hearing the other side of the conflict, this whole thing could mean multiple things. It's possible that the manager himself was a bad culture fit, for example. If the manager wasn't actually a bad culture fit and his managing clashed with the author of this blog, then the author is actually a bad culture fit for Spotify.
This is exactly the sort of problem I can easily see brewing with remote-only development. Conflicts escalating in DMs, opinions stewing on both sides because if limited interaction, and no good way to get body language across that might help add understanding to the situation.
Telling your manager to go read a book on managing seems like a terrible idea. I don't know in what mindset this is acceptable. Using sarcasm and mocking someone remotely is also a terrible way to get your point across.
Then there's the Kanban board thing. It's great that you communicated the problems you've been having in the sprint system. It's great that you came up with a working solution. What's not great is implementing this behind the manager's back. You can't go "oh yeah we changed the way we do this" to your manager and get defensive even if the team agrees with you, you should've taken his opinion into consideration before you even came up with a solution.
Questions like "how long do you expect x to take" seem pretty normal to me. As a developer it's obvious that there's no real answer to this that will satisfy everyone, but companies need long term strategies and predictions to shift resources around. Dealing with this uncertainty is a manager's problem but asking for an estimate is hardly the unacceptable crime against dev teams that this blog makes it out to be.
Reading through the emotional descriptions of what happened, it seems to me like both parties were heavily affected by covid, lockdowns, and forced remote work, but neither side seemed to get much of an understanding from the other.
I think it's unfair to blame either party for this. I don't think there's anything wrong with publishing your experience per se, but come on, put it up on a blog somewhere, don't dedicate an entire website to the story of how you got fired. That just seems petty.