My guess: the 60 messages per second is an average. Monday morning in the US timezones might see several multiples of that. Weekends could be a lot lower.
It's definitely very bursty. On weekends and holidays things are much more quiet. During peak load we'll be in the hundreds/sec. Also keep in mind that chat messages don't actually make up the majority of the traffic we serve; it's presence information (away, idle, available), people connecting/disconnecting, etc. (I'm one of the HipChat co-founders.)
And? The article makes no mention of what each server is responsible for, how much redundancy is built in (this is AWS where nodes die on a whim), whether all of these servers are used for production or if there is a spread across dev and staging environments, etc.
When all of these factors are considered, 52 servers isn't a huge number.