Annie def got super lean between 2020 to 2021/2022 if you look on Instagram. Fiona has always been small, but in the last year to two has definitely gotten leaner. Lost some essential body fat for sure. It's working now. But I agree with your "look test". I see it in runners right during the height of their career (unless they make a comeback) and before major career ending issues happen. Migration theory can explain how they run so well for a period of time. It was probably beneficial back when food was scarce to have some individuals thrive (or being able to move a lot) despite being under fueled. They were the ones to find new places to migrate to, and hence find more food. It cannot last forever though and usually only works 1 time. This is just a theory though.
But maybe it's worth it to her. She just made the Olympic team, but who knows about her longevity. I've seen this too many times and 99 times out of 100, it doesn't end well.