All these traits go beyond just increasing the odds of survival, they improve the life of the individual directly. I.e. they confer fitness.

Wrong. Fitness is not "beyond" increasing the odds of survival, fitness is ONLY that. Fitness is not about quality of life.

- in that ecosystem they are inhabiting, be more healthy than those who don't.

Survival of the fittest says nothing about how healthy they are, only whether they survive and reproduce or not.

So it should really be "If it survives it must have been fit"