Overwatch does indeed use Raw input, which ignores windows pointer ballistics, including EPP.
Now, one of the big myths about raw input is that it is actually RAW INPUT from the mouse hardware, and this is not actually the case. It is still subject to a number of different OS level modifications including DWM compositioning and system-wide DPI scaling (even in fullscreen exclusive!). This is fairly easy to prove by using the “application compatibility” settings. If you enable/disable per-process-dpi or HighDPIAwareness, your input response will change slightly with each setting. If raw input actually read mouse X/Y deltas directly from the mouse, this shouldn’t make any difference.
All raw input really does is bypass windows pointer ballistics - which is the “mouse settings” on your control panel, including the slider for mouse speed and the enhance pointer precision checkbox.
You can test the effect of EPP yourself by turning it on/off and comparing both settings. If it were active, it would make your input drastically different. Try the same test again with a game like CS:GO or aim-lab that allows you to select “windows input” instead of “raw input” and you will see how much differently it moves with EPP on vs. off.
In general, it is advised that you should have it off, but you are correct that it shouldn’t really matter for raw input games.