Good content! Disabling buttons is always difficult to thread with accessibility. As Aishwaryalakshmi mentioned below, it removes it from the accessibility tree.

So, when a screen reader reaches a page with the form, it will call out the input controls (including buttons). But if it's disabled, it'll never receive focus without valid input. Unless the developer has used some well-written aria-live attributes, the user will never know that the button has enabled.