That used to be true. But TFA points out it's not true now. Ebola used to be too lethal to be efficient: It killed its host so quickly it didn't have time to spread. But because this outbreak is in urban areas, it does have the ability to spread. Which means it has many more opportunities to evolve.
>Which means it has many more opportunities to evolve
More opportunities to evolve doesn't mean a rapidly mutating virus, relatively speaking. It may be mutating faster now because of more opportunities, but the virus still mutates orders of magnitude less rapidly than influenza.