Where specifically are you getting that information? From TFA: The nitrogen atmosphere, in which the sample was stored during this time, can be a bactericide

Where specifically are you getting that information? From TFA: The nitrogen atmosphere, in which the sample was stored during this time, can be a bactericide for some, but not all, Bacillus species owing to the reduced water activity associated with the dry atmosphere (Munsch-Alatossava & Alatossava, 2014). The bactericidal effects of such atmospheres are not restricted to Bacillus, and thus provides no diagnostic information. That is: nitrogen should generally kill bacteria, but may not in the case of some Bacillus species. That's quite some distance from saying the bacteria would thrive in that environment, and I'd presume other factors (light, food supply, some means for oxidation) would be necessary, most of which could be reasonably constrained within a sample-examination environment. Origin determination might be made through C14 or other dating --- any extant in situ bioactive materials would presumably be comprised of primordial carbon with effectively no C14 signature, as opposed to any recently-deposited or growing organisms. One challenge seems to be that there was very little Bacillus present, 11 to 147 individuals, which seems to have made DNA analysis impractical and would likely challenge isotopic analysis as well. NB: Not my area of expertise, just close reading, general understanding, and some research-fu.