I know it's probably not satisfying, but they spent 10 years on this research asking this very same question. If the only remaining possibility is biological origin, then we should probably take the risk of ignoring an unknown unknown.
I don't think such approach is fully consistent with scientific methodology.
The abstract clearly presents the question as an alternative, not "the only remaining possibility", as you stated:
"PH3 could originate from unknown photochemistry or geochemistry, or, by analogy with biological production of PH3 on Earth, from the presence of life"
It doesn't strike me as scientific to "ignore an unknown unknown", which would boil down to an assumption that we already know every possible reaction mechanism there is to know.
Discovering unknown chemistry would be groundbreaking of course, but the same goes for confirming extraterrestial life.
Indeed not 100% pure science, but I like the fun side science too where I can just forget about the rigours and entertain ideas that are likely to be true.
I've just read the paper and all this is right. Before that I haven't seen these claims so we are now in maybe life territory again. Sigh.
