Strug had a severely injured ankle and yet she did her vault knowing that she had to nail the landing and put serious stress on her injured ankle. From what we can tell, Biles his physically ok. If she felt mentally off, I think she could have/should have gutted out the final three events and even dialed back her difficulty level a bit if she was worried about her focus.
Just two days prior she got the best combined score in preliminaries. She could still compete ok. So if it comes out that there is no physical problem with her, then yes I am disappointed that she did not finish the meet.