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.
After this the billing department might just get a list of companies that are behind and start a process.