Taskbar buttons absolutely benefit from Fitt's Law.
Fitt's Law says that the time required to interact with an element depends on both the distance to that element and the width/height of that element.
Putting an element on an edge (where the cursor cannot go beyond that edge) essentially makes it infinitely large in one dimension. Putting it in a corner makes it infinitely large in two dimensions.
Taskbar buttons in Windows (or menu items in macOS) do benefit from Fitt's Law, just not as much as items in the corners.
Fitt's Law says that the time required to interact with an element depends on both the distance to that element and the width/height of that element.
Putting an element on an edge (where the cursor cannot go beyond that edge) essentially makes it infinitely large in one dimension. Putting it in a corner makes it infinitely large in two dimensions.
Taskbar buttons in Windows (or menu items in macOS) do benefit from Fitt's Law, just not as much as items in the corners.