Any number of undiagnosed conditions could cause systemic tiredness weakness or fatigue and many of those conditions can spontaneously resolve. Having fully developed ME and soon bouncing back into competitive athletic shape just doesn’t add up.
"add up" ... for the typical course of the disease.
There have been no (?) rigorous studies of what the normal disease process is in people with CFS/ME in the long-term.
Do 50% of people recover to pre-morbid functioning soon after being badly affected - certainly not.
Do 1%? We have no idea.
ME/CFS is also a diagnosis of exclusion.
Viruses can - in some cases of people with rare genetic mutations cause specific very odd symptoms in people.
We know that ME/CFS is not genomically linked simply to anything, but know much less about specific subgroups of the population that might get rapidly better.
ME/CFS seems likely to be a dysfunction of the immune system. The right 'perfect storm' of odd conditions might snap the immune system back to normal, for some patients.
I don't think it's very safe to say much more than 'did not follow the typical course of the disease'. And it'd be better to say that in 50 years time after we see she diddn't relapse.
