It wouldn't be too surprising, if this were true. Mobipocket supposedly has exclusivity clauses in their contracts that say if you are supporting Mobi DRM on a device that you are not allowed to support any other form of DRM. iRex may not have a choice if they want to start supporting Adobe DRM and Mobipocket is unwilling to budge on that clause in the license. iRex may only be able to pick one or the other.
I asked iRex this specific question a while ago in their forum about how they were going to support both, and the response was "no comment".
IMO, it's a little early yet, since there hasn't been anything official from iRex, but it certainly sounds like the speculation is correct. Whether they remove Mobi completely, only remove Mobi DRM, or offer two versions is yet to be seen. There could be another option that we don't know about yet.