This is all fairly plausable, if you accept the hypothesis that life either did not start here, or was started elsewhere as well as here (and resulted in similar creatures). To fossilise a creature, the most common way is for it to die in a solution of salts, like groundwater here on earth. The cells suck in mineral salts as they die through the failing cell walls, concentrating the minerals. Ceres, the largest dwarf planet sits in the asteroid belt, so is presumably getting the shit kicked out of it 24/7, but is a bit too cold for liquid water, but other (admittedly far more complex) solvents would still be liquid some of the time. TBH if there was life on Ceres at some point, I'd have expected us to have more than half a dozen asteroids from it with bacteria on, but the theory doesn't preclude planets in other solar systems.|
Saying that any of these apparently earth-like bacteria are extremophile enough to survive in interstellar space is another leap though, but if this evidence were to indicate extraterrestrial bacterial life, it would increase the odds just a touch.
Of course, this news report is extremely premature. It hasn't even been published in a journal yet, so it certainly hasn't been peer reviewed, and the most likely explanation of this sort of find (having not read the report) is failure of clean room techniques, I'd say.