Isn't there a meaningful sense in which "separate VMs for different apps" constitutes a cluster?

The "cooperative task" they're engaged in is just, broadly, meeting your needs, whatever they are.

The isolation is a desirable property, and I agree this is much preferable to a highly inter-coupled bunch of machines, and also that thia stretches the typical sense in which we refer to a "compute cluster", but I don't think it's an entirely invalid framing of the term.