Evolution is survival of the fittest. That's not a tautology, it actually says something, namely that the traits which survive and thus propagate tend to be the ones that enable some form of adaptation to its living conditions to the individual. The paper lists a bunch of examples:

  - lactose tolerance
  - immunity and disease resistance
  - lighter skin at northern latitudes
  - metabolism and vitamin D processing changes in response to changes in diet after the rise of agriculture

> 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"