Just because a fossil looks similar does not mean it hasn't evolved. Most evolution happens on the molecular scale, if you looked at the genomes I guarantee they would be different.
The paper in PNAS discusses this at length. It clearly states that it would be very nice to be able to check DNA, but that defining species in microbes is about phenotypes, not genotypes, and the important thing is that there is no sign of speciation (that is, two separate populations, separated in space, did not diverge.
The morphology-based “concept of hypobradytely does not necessarily imply genomic, biochemical, or physiological identity between modern and fossil taxa," a claim of extreme evolutionary stasis—a lack of speciation over billions of years—would be strengthened not only by discovery of additional fossil communities but by firm evidence of their molecular biology. Although speciation-based evolution occurs at the phenotypic rather than genotypic level of biologicenvironmental interaction, the biomolecules underlying such change are not preserved in the rock record in which such assessment can be based only on indirect proxies and inferences of physiology based on isotopic analyses
However, the article noted, it's possible that this interpretation is wrong and that what they saw was two separate populations that underwent convergent evolution rather than one population that was separated and remained static or that there might have been significant biochemical evolution that did not change the morphology.
Moreover, large-diameter (“giant”) sulfur bacteria of differing phylogenetic lineages can exhibit similar morphologies and patterns of behavior suggesting convergent evolution of morphologic “look-alikes” adapted to a same or similar function. Although it remains to be established whether such morphological “mimicry” is exhibited also by the more narrow 10-m-diameter sulfur bacteria described here—the two modern sulfur bacterial taxa of similar dimensions being aerobes rather than anaerobes like the Duck Creek and Turee Creek fossils—it remains conceivable that the marked similarities between the two mid-Precambrian communities and their modern counterparts could be an example of the so-called Volkswagen Syndrome, a lack of change in organismal form that masks the evolution of internal biochemical machinery