The buttons do highlight when you move over them, so it's entirely clear if you hit them if you would click. Or is that not what you mean by the 'affordances' of these buttons?
This means the user has to roll the mouse around to discover what's clickable. That's not usable, it's frustrating.
In the case of the tool buttons in macOS windows my feeling is that this is really is splitting hairs. Literally everything that is not the same color as the toolbar itself is clickable. The frustration or the supposed 'cognitive overload' is purely theoretical.
Ah then that would mitigate the issue a bit. It would obviously be better for pointe targeting purposes if the button edges are visible ahead of time so that users know where it's acceptable for their pointers to land, but having them show up when the pointer hovers over them would help some.