By "same platform" they mean same capacities/capabilities, ie. ram, disk, processors. They don't care whether virtual or not, just that you don't try to pair a juggernaut primary with a minuscule secondary and then expect that minuscule secondary to somehow cope when the juggernaut goes down.
The bit missing from the (I agree confusing text) is that if you intend to use resilience features then both platforms need to have enough additional capacity to support the others users and extensions during resilience.
There is a comprehensive capacities document that was release this week:
Stuck in a never ending cycle of file copying.