Was there ever a problem downloading a KEPUB, whether based on EPUB3 or not?
I thought the problem was downloading the ADE-compatible EPUB3 (which doesn't use oboK anyway).
As for whether or not Kobo has enabled hardened DRM on the Adobe Content Server, thus necessitating ADE3/4... probably not, simply because retailers don't seem to be brave enough to take the jump, despite demanding the upgrade in the first place.