I'm not super clear on how this is difference from using any kind of wiki like Confluence. Is there something specific about the format or use of P2 that makes it better for distributed teams?
Just like on HN, one comment deep in the thread can be way more valuable than the original post. (I work at Automattic)
P2 is for discussion and synchronization across time zones. It also involves a large amount of cross posting between p2's to involve other teams, divisions, and areas of expertise.
P2 replaces work email. It's open by default, has better formatting and comes with github integration. I haven't sent a work email in months.
Slack is for low value, ephemeral chatter.