So, my understanding (layman who reads about these things), is that, long ago, one particular Tasmanian devil got cancer. This devil bit another and transferred some cells, which grew on the recipient. Note that these cells belong to the original donor. ALL of these cells, on all of the affected devils, genetically belong to the donor. It's literally a separate parasitic life form that reproduces parthenogenetically. Like "Henrietta Lacks", the cells are still alive, long after the original being is gone.

So, none of the recipients cells has cancer. They have someone else's cancer living on them!