To be fair, the overlap may easily be larger than the figure implies, but you can certainly describes properties without having to partition a function’s domain.
In fact, the canonical example of property-based testing (that reversing a list twice yields the original list:
reverse (reverse xs) == xs) does not rely on partitioning. It works for all finite lists.
You may think that this is only because the case is so simple, but that’s not the case. You can also avoid partitioning on the slightly more complex problem presented by the Diamond kata. In fact, the domain for that problem is so small that you don’t need a property-based framework.
This is an interesting look at two related but separate branches of testing.