I don't think it has a single thing to do with Slack... Atlassian's main product is not Hipchat, it's Jira. A product which thousands of large enterprises use and pay a ton for. And has nothing to do with Slack.
It's price to the enterprise is seen as almost zero from their perspective. The other products that solve the same problem cost 10x as much.
The entire Atlassian business model was disruptive. While their competitors were focusing on buying steak dinners for the highest exec that their sales team could get in contact with Atlassian products moved organically through the company. Awesome user groups cheap prices and a strong developer ecosystem made them very difficult to compete with.
One of the complications with trying to compete with disruption is that people inside your own company don't want to realize they are being disrupted. You see plenty of "the old plan is failing because we have not put enough time and energy into it".
