At any rate HipChat was already dead prior to the sale. Atlassian made a new program from scratch called Stride which was its own little awful can of worms.
> It was like 25k people in a single room iirc.
This never happened. I literally could not have happened. Uber ditched Hipchat when it was around 10k employees. The largest rooms I ever saw were outages or eng rooms with a few thousand, most with the room muted unless they needed something.
Seeing this behavior was as easy as doing a join in mysql for user ids and rooms. I mean it's likely you wouldn't have seen it, or the room, or had knowledge of it unless you weren't directly involved with whatever it was?
Most open-source and commercial software advertise that they can scale, but throw the first 500 or 5000 users and it's breaking apart.
Build it, and they will come ... and do something you hadn't planned for.