As I understand it, the big deal about this discovery wasn't the presence of phosphine, but the amount of phosphine they found. The authors of the original pap

As I understand it, the big deal about this discovery wasn't the presence of phosphine, but the amount of phosphine they found. The authors of the original paper went through all of the well-known scenarios for generating phosphine abiotically, and reasoned that the concentrations these would produce were too small for them to be the cause. That left either a biological mechanism (most likely microbial life in the atmosphere), or some unknown abiotic mechanism. Edit: Fixed typo, phosphate -> phosphine.