KDE moved because Phabricator is practically abandoned and KDE can't take it over.
Furthermore, all KDE development is based on git and Phabricator abstracts the versioning systems, making the naming of things weird. They require weird cli integration (arcanist) not to rely on git semantics.
Most contributors expect gitlab/github kind of workflows on the other hand. It's really not great to make newcomers learn how to use arc (which is really not that good and barely used elsewhere) when they can learn proper git commands that will be useful for them in many aspects of their professional life, or just reuse the knowledge if they're already familiar.
I understand whoever wrote the article didn't want to be complaining about Phabricator as much as talking about how good gitlab is.