As Harrison noted, the authors are appropriately conservative.

The team found that the carbon trapped in a handful of rocks probed by the rover is dramatically enriched in light isotopes of carbon. On Earth, the signal would be seen as strong evidence for ancient microbial life...

...however, the researchers are reluctant to make any grand claims, and they have worked hard to concoct alternative, nonbiological explanations involving ultraviolet (UV) light and stardust. But those alternatives are at least as far-fetched as a scenario in which subterranean microbes emitted the enriched carbon as methane gas. The team concludes the study does “inch up the plausibility” that microbes once existed on the planet...