I believe I am a pretty good source. :)
Stride is a brand new product (not a rebrand of Hipchat), we did not repurpose Hipchat's code. It's built from ground up on Atlassian platform - new protocol, clients, platform, etc. One exception is the video bridge technology which remains powered by the Jitsi.org open source product (which we acquired in 2015). I do look after both products (Hipchat Cloud and Stride) for Atlassian, happy to answer questions.