> accessibility is just usability. all it is is making sure your product is as usable by as many people as possible.

Not always. There are things that increase usability for fully abled people but decrease it for disabled people. Color-coding is a common one. Another is using smaller font size to make more info visible at once, or advanced touch gestures that can be accidentally triggered by shaky hands.

Then there are accessibility features that actively harm fully abled users, like sticky keys or screen readers that automatically follow focused UI elements.