No. Hipchat had an on-prem option. In fact, I'm kind of stunned to see that the blog post on Slack's website doesn't mention the on-prem version at all. Given that on-prem was one of the differentiators that Hipchat had against Slack (and one of the reasons that many companies stuck with Hipchat, even though Slack had more features), the lack of discussion around the on-prem version seems like a huge mistake.
Hmm mentioning it would probably be a bigger mistake. If they say anything they'll have detractors and proponents, by staying silent it allows them to prep & bleed off users who MUST have onprem.
All in all I would guess that slack will probably retain more than 80% of users, which is pretty good.