I'm in no position to say definitively one way or another whether XeniumOS is based on Cromwell or not - I'm just going off of what I was told by a rep from Team OzXodus.
I was also told that the guys at OzXchip were working on OzXchip II. For whatever reason they went their separate ways. Part of the team joined up with Xodus, formed OzXodus, and released the Xenium, which uses a codebase originally written by those team members but finished and improved upon by OzXodus at a whole. The other OzXchip team members went and joined the PCBioXX/NitroXX team and released the SmartXX, which uses the same codebase taken to OzXodus, but of course this codebase was finished and improved upon in a separate manner by Team SmartXX.
So, in short, my understanding is that they both originate from the same codebase, but the XeniumOS and SmartXX OS diverged when they were brought to two separate teams. As far as the mud-slinging, accusations of "stealing" and such go...I would take that with as much gravity as I would a divorced couple arguing over custody of their children.