> Do they offer on-prem servers?

Yes.

> Then how does it bring cloud scalability?

You buy an entire (or half) rack at a time, and then can treat the entire thing as one elastic pool of resources, you do not need to manage the individual sleds within the server yourself. Your interface to the rack is similar to a VPS provider, it's "give me a new vm with these specs."

Happy to elaborate if you have more questions.