I think it's debatable, at best, whether Occam's razor would favor "an unknown process" over a known biological process. Especially when some research apparently has already been done, without success, to discover alternate processes.
(And, by the way, the unknown process could also be biological!)
You're implying we don't have any information at all. We do.
Even if you argue is it's limited, to our knowledge bio processes outside of earth has been zero. Whereas, non-bio chemical processes is greater than zero. So now people want to bet on that it's bio??
Milky Way contains on the order of half a trillion star systems. Would having, say, a million planets carrying some form of life in our galaxy alone qualify as “rare”?
To be fair, I wouldn't consider that rare. When we talk about life, we usually think in terms of are we alone. If there are a million planets with life, life isn't rare, regardless of what percentage of planets have life.
You don't know if an unknown route to phosphine generation would be simpler and more likely, you would just find such an announcement more mundane, which you mistake for it being more plausible.
Invoking Occam's razor to compare two unknowns makes about 0 sense.
I think an unknown route to phosphine generation seems simpler and more likely than a biological explanation. YMMV.
Occam's razor would therefore imply that this is not life.