I think comparing feature for feature isn't the right way to understand why Slack is a disruptive innovation.
Often it's an innovation along some other dimension - process, business model, etc. In this case it's Slack's business model and "platform" strategy. Other tools like HipChat were just that, a tool. Slack is a platform in the sense that it enables businesses and developers to leverage and extend the platform, thereby adding additional value to the platform. This leads to so-called "Network Effects" where as more apps are built on the platform, the more valuable the platform and the more likely others will want to build on that platform, etc. in a virtuous cycle.
Often it's an innovation along some other dimension - process, business model, etc. In this case it's Slack's business model and "platform" strategy. Other tools like HipChat were just that, a tool. Slack is a platform in the sense that it enables businesses and developers to leverage and extend the platform, thereby adding additional value to the platform. This leads to so-called "Network Effects" where as more apps are built on the platform, the more valuable the platform and the more likely others will want to build on that platform, etc. in a virtuous cycle.