As it is written, the Sub button should force any inserted mesh to be a negative mesh, whether you pressed ALT or not. It currently does not do this. It behaves just like the ADD function. It serves little apparent purpose at present, as it is redundant.
The Add button is really only there as an alternate UI push-button way of accessing the Ctrl-Drag dynameshing action, the same way many functions have buttons, but also hotkeys or context sensitive gestures…for instance you can clear a mask by ctrl dragging in empty space, but also by pressing a UI button. It is the same as simply remeshing by ctrl-dragging in the canvas.
What may be confusing you, is perhaps you are thinking of the buttons as modifers that would modify the ctrl-drag remesh function (I know I did at first). By they are manual functions that can be pressed after Dynamesh mode is activated. It’s really not that complicated once you understand it, but I agree there does need to be a way to force tag any mesh as negative, and hope to see this eventually.