There is about as much ownership here as a squatter in a two-bedroom apartment. They are apologizing because they got caught, not because they genuinely believe they messed up.
No, I’m pretty sure Slack was never a “not for profit” company. Most not-for-profits do not ambush their customers with $250k bills on a week’s notice. I’ve seen debt collectors less predatory than this.
May I suggest a public post mortem so that the doubters can see that you went after the root cause?
Might be interesting for HN as well as big companies have mistakes in policy, teams and automated collections...