It seems to me there are several red flags in this report.
e.g. the paper does not report phylogenetic relationships between the reviving organisms and known bacterial phyla. The closest they come is reporting a bit about the in situ community in the mud. (I do not see them rule out the possibility that what *grew* was from contamination. Never mind the acknowledged difficulty cultivating many environmental microbes, and they claim huge success.)
Similarly, bacteria are motile... and mud is wet. 100 million year old sediments, even those 75 meters deep (or whatever it was), can be squiggled into by a determined cell from higher (younger) in the sediment.
The paper may indeed be reporting something important, but ... meh. Not much to write home about.
Sincerely,
Your local environmental microbiologist grump.
p.s. To be clear, I have no doubt at all that bacteria, and archaea, and perhaps protista, can persist, dormant, for remarkable lengths of time. I see no reason why they wouldn't. Life evolves to be stable. I'd like to see an in situ phylogeny, and a phylogeny of the organisms they stimulated, that's all.