I'll give you my take on the article.
kdbus is supposed to replace D-Bus. Can this be confirmed somehow?
That is explicitly the intent, but WRT "confirming" it, there's no central authority that can say, "Yes, here is our timeline for the future of GNU/Linux" -- beyond the kernel, that's a heterogeneous and de-centralized realm.
Of course, seeing as how it is the kernel, a lot of decision makers in that de-centralized realm will probably be interested in cooperating. It sounds like a good thing.
will be an easy task
I don't see any indication that they can't both be used at the same time -- which would be by far the sanest form of transition. So "easy" depends on your context...
imagine that both have a distinct API?
The Greg K-H announcement linked at the beginning refers to a userland compatibility layer, which is also very sane with regard to transitioning; initially some distros could make both available, others move directly to the compatibility layer, etc.
Sometimes it is good to move forward in a way that sacrifices backward compatibility. Consider perl 5 vs. perl 4 or python 3 vs. 2; within the major version, improvements (indicated by new minor versions, 5.8, 5.9, 5.10, etc.) are made with backward compatibility a priority, and meanwhile work on the work on the next major version (which is incompatible, but presumably much improved based on experience with the current version) may be progressing.
Distros that have versions are similar in the sense that while the current version is being maintained and updated work on a new version is also going on. This makes the incorporation of fundamental changes like kdbus easier. I guess we'll see what happens.