"Hybrid cloud" is being able to move applications back and forth between on-prem and cloud, and scaling from on-prem into the cloud if need be, typically enabled by the proliferation of k8s on both sides of the equation.
The difference between that and your description is that it's not merely "mixing different types of servers" where each type is managed in a completely different way and running completely different applications. You're obviously correct that there would be nothing particularly special about that.
Usually, but a mainframe is an on-prem solution and can only fill that part of a hybrid cloud strategy. I get the feeling IBM thought they'd lose the CTO crowd if they didn't say "cloud" every now and then.
And good that they do, otherwise I would not get so much evergreen enjoyment from the cloud-to-butt browser extension I've had installed since forever.
I agree with the parent post in that it's mostly marketing. There was nothing preventing you from calling back and forth from cloud services on your mainframe before the recent marketing shift to "hybrid cloud". Hybrid cloud is a meaningless term on a technical level, but I guess for buisness people it means you can buy a new mainframe and that won't stop you from delivering "cloud" or whatever other initiative the suits have budgeted for.
So yes, its laughable to hear that kind of marketing if you are technical. But for the kinds of large organizations that would potentially be buying a new mainframe, its what you need to get this sort of thing budgeted.
Certainly the IBM systems group didn't come up it. They know just as well as the people trying to upgrade their mainframes that it's an on-premises strategy primarily intended for line of business applications with reasonably well understood resource utilization.
