Slack has a rather large number of paying customers for a messaging platform, so even over just 10 years you could extract quite a bit of cash directly on top of that owning platforms allows for a lot of indirect profits.

That said, if they make ~15M per month directly then up to 1 to 4 billion is probably completely reasonable purchase price. And depending on how things are evolving and internal costs etc 9B while probably high is not crazy ridiculous.