The thinking seems to go like this:

1. phosphine is found on earth in low-oxygen environments that harbor life (e.g., swamps)

2. abiological routes to phosphine, although known, require unusual reagents and/or elevated temperatures

This 2019 article gives some background:

https://news.mit.edu/2019/phosphine-aliens-stink-1218

Here's some evidence of phosphine production in a swamp:

https://www.sciencedirect.com/science/article/abs/pii/014663...

This leaves (2), which is a bear because it states a negative. The claim that the phosphine on Venus is biological in origin requires no feasible route to the substance given the known chemistry of the planet's atmosphere.

Given that Venus' atmosphere is a complex mixture held at very high temperature and pressure, the "sign" seems pretty weak here.