Seems a bit weird that it's only the timing window that improves..
I don't know. You can hit as perfect shots as possible, but still, if you're not in perfect position for the shot, you don't get a "perfect" shot even though your timing is perfect. With TI, SI and SC, it seems that you're able to create much better shots of bad positioning too. If it's only the timing windows that are larger, they have to be veeery large to account for the bad positioning(so if you hit perfect from a bad position with TI/SI/SC, you would get "super-perfect" ), and that don't fit with just having a bigger timing window, because then no matter how bad your timing is, you should be able to come up with decent strokes. Meaning that you would be able to hit decent strokes of very mistimed shots(like when you're waaay "too soon" or really "too late"). However, that is not the case. If you mistime it totally, you don't end up with a half decent shot, you end up with either a unforced or a really bad shot no matter what skills you have. Therefore, if the timing window is larger, you would be getting a "timing-bonus/super perfect" shot if you hit perfect, meaning that your well timed shots with one of these skills are better than well timed shots without. Meaning that you get better shots with the skill, if your timing is somewhat decent. Meaning that you can't make shots like these with coaches who don't have the skill(you can't hit "super-perfects" without one of these skills). Meaning that they aren't fair (especially top spin shots, because they're so effective in the first place).
Does this make any sense to anyone?
My point is, that I don't think it's just the timing window. It seems like a valid idea that it's just the timing that increases. However, I think these skills also help with improving the shots generally. Therefore, often making impossible shots from bad positions/out of position/reach-positions.