Not necessarily. A trait may also be there simply because it's not actively harmful, or maybe because it's a useless side-effect of another trait that is beneficial.
It may be that zebras have stripes because they are not harmful enough to prevent them from passing their genes forward. (of course, in this particular case the flies hypothesis seems more likely).
You don't know that. The article doesn't even bother to test it.
They've observed that stripes lead to fewer horsefly landings. Interesting. I guess.
But, talking about evolution is a non sequitor. Show me the data where a zebra's fecundity is related to the number of horsefly bites it receives. There isn't any.
One can speculate about such things. But, if one wishes to be scientific, test to see if there is a measurable effect.
For all we know, zebras with MORE horsefly bites are MORE likely to reproduce. There simply is no data in the article one way or the other. So bringing up evolution is just mindless unscientific just-so-storytelling.
Doesn't it sound like such diseases are likely to alter a zebra's fecundity?
As far as the effect of 'such diseases' on a zebra's fecundity, I have no idea. For all we know, a feverish lethargic zebra is EASIER for a stallion to mount than a healthy zebra. Nobody knows.
The study, incidentally, did not deal with the types of flies accustomed to biting zebras. I have every reason to think a typical African biting fly would have no problem alighting on a zebra... it's what they do for a living.
Zebras have stripes because their stripes shoo away flies, which helped previous generations of zebras who had stripes thrive and pass their genes forward. Why do zebras have stripes? Because they shoo flies away and evolution ensured that that helpful trait kept passing through descendants until every zebra had it as a dominant, visible trait.
No one ever said there had to be "someone" somewhere to paint zebras black and white, that's just you using anthropomorphized evolution as a straw man/entity.