Perhaps a better or just another way I can say it:
Nearly all of my Device (“Thing”) related discussions are protocol agnostic.
Thus, since a ZigBee Device cannot be directly associated with a Z-Wave device, the concept of Association does not apply in my context. Ever.
Yes… as per your documentation reference, there are associations for special cases within a protocol. But I focus on the abstraction of Things independent of protocol, such that the end-user and SmartApp developer properly can ignore the communication protocols and other physical attributes of things, and only deal with the abstraction provided by the Device Type (ideally abstracted to the standard SmartThings Capabilities.
It’s not about being right or wrong – it’s about context.