"leaving the tantalizing possibility that we’ve detected the last vestiges of an ancient ecosystem."
According to the paper in your first link [0], PH3 gets destroyed by ultraviolet light-related chemistry, and so has a short lifetime in planetary atmospheres (section 2.3 on page 7, though that's specific to Earth's atmosphere). I assume this means if they've detected phosphine, there must be an ongoing process generating or replenishing it.
Perhaps possible Venus' very thick atmosphere shielded it from UV? Or a large pocket of it was recently released due to gelogic activity. Just spitballing, I don't know enough about phosphine chem to know if that's feasible.
According to the paper in your first link [0], PH3 gets destroyed by ultraviolet light-related chemistry, and so has a short lifetime in planetary atmospheres (section 2.3 on page 7, though that's specific to Earth's atmosphere). I assume this means if they've detected phosphine, there must be an ongoing process generating or replenishing it.
[0] https://arxiv.org/abs/1910.05224