Well they're quite clear that it's on premise, so under their direct control.
I totally agree with you however, WTF is meant by cloud in this context. Smacks to me of a PR bod using buzz words to garner some free column inches. There's probably a NATS IT bod squirming somewhere.
In fact what is probably happening is that NATS will replace some old onsite infrastructure with some new onsite infrastructure, probably deploying machine virtualisation so that they can scale up capacity super fast in the event of a spike in load. This would make sense given the last failure resulted from overloaded server hardware due to a spike in load caused by an unforeseen use case of the NATS software.