Because of the extreme nature of the Venusian environment, if you fully read the study, they spent time evaluating the possibility of a natural source. Although it can be produced by volcanism, etc it is the volumes of the gas that point to life as the source. It also follows a pattern of following a temperate temperature range of the clouds. If it is from another source, the biochemist/chemist cannot come up with a mechanism for it. It’s all very intriguing.