I don't think Simone Biles has been on Strictly, has she?! I don't watch it! She's not the only Olympian to be doing significantly less training than some of these contestants though. Cyclist Kristin Armstrong apparently does 20 to 25 hours a week on her bike and Olympic skier Sara Renner does about 25 hours of practice. One study that looked at endurance athletes competing at the highest levels and found the average was 13 to 17 hours a week. Some of these Strictly contestants were doing almost that in a single day. And while for some it does sound like a choice, for others it definitely doesn't. Laura Whitmore didn't appear to have chosen to do that level - it sounds like she was pressured into it to some degree.
Re dancing being less taxing than gymnastics I'm not sure that's true. There have been several gymnasts on the show. Louis Smith did Strictly 2 weeks after doing the Olympics. He said:
“Within the first hour, I was laying on the floor with my ankles and heels in bits wearing Cuban heels.
“Flavia was just kind of looking over at me thinking, ‘How can you call yourself an Olympian?’
“So yeah, it really was a completely different type of energy that you had to use and a different type of fitness.
I still think the diagnostic criteria could have been interpreted very liberally, if you take the risk of serious injury to mean a stress injury like PTSD or one of it's sister conditions (there seem to be others that are essentially the exact same symptoms and presentation but without having the need for the horrifying event - PDSD for example (Prolonged Duress Stress Disorder), Cumulative Stress Disorder etc. Although who knows. Maybe there were other stressors in RR's life as you say or maybe she got the diagnosis abroad where the criteria might be different? Her partner/husband is Russian is he not? We will probably never know.