100%. Even without an MRI showing a stress fracture, Team USA's physios surely could diagnose it with 95% certainty just by moving her leg around to evoke pain.
Fiona O'Keeffe does not look like someone on her first stress fracture, and anyone who has had a stress fracture knows that you can't run a marathon with a stress fracture.
Also, the way she moving looked like it's a pretty advanced stress fracture, meaning she's tried to run on it several times since first noticing pain. I had a pelvic stress fracture once. The progression went like this:
Tuesday: Noticed nagging pain in a cooldown after a hard track workout.
Wednesday: Did a 14 mile medium-long run at my normal easy run pace; pain was present and nagging but did not affect my pace or gait
Thursday: 8 mile morning easy run, still with normal gait and pace. The pain was slightly worse than Wednesday. Evening threshold workout. The pain was very bad in the warmup but I noticed it less while running faster. The cooldown was also very painful.
Friday: Another 14 mile medium long run. The pain was very bad, but my gait felt nearly normal. My pace was affected only slightly.
Saturday: 16 mile easy long run. I felt like I was forcing my gait to be normal and the pain was intense. My long run pace is 30-45 seconds slower per mile than usual.
Sunday: 8 mile 'easy' run. I was noticeably limping and and running much more slowly than usual.
Monday: Made it 5 miles before I had to limp home. I was still less injured than Fiona looked Sunday.
My point is that by Sunday she was clearly nursing an injury that had been worsening progressively and had plenty of time to withdraw. I have no doubt that she has a stress fracture and has been aware of it for a while.