Hopefully if they can get a probe out there they would be able to tell definitively if it was Soviet contamination or not, although I'm not sure how they would be able to do that. I could see them ruling it out (some entirely different DNA structure or something) but not sure how they'd say 100% that it was Soviet in origin.
Even if spores did survive the space trip (actually quite plausible, [0], venus trip is ~110 days), the atmosphere is extremely dessicating and acidic (SO2, SO3, H2SO4), and the ground is too hot for any known life. The dessication and osmotic pressure is the real kicker. It's very unlikely spores would germinate without rehydration.
[0] https://www.nasa.gov/mission_pages/station/research/news/eu_...
[1] https://www.nytimes.com/1966/03/06/archives/soviet-says-venu...
So not only do we have no good explanation for phosphine, we do not have a good explanation for the unknown absorbers. Well, we do have one good explanation although it seemed unlikely and that is life. Given that this is second effect that could be explained with microbes I feel it adds that much more weight to the theory.
This bacteria is introduced to an environment that has all it needs to spread quickly. There's absolutely nothing out there that it has to compete with. There's nothing out there consuming it or infecting it or otherwise inhibiting its growth other than a lack of whatever nutrient it consumes.
Rabbits went from 0 to over 200 million in under 200 years in Australia, and that's with snakes and disease. Apparently they spread across the continent in just 50 years.[1] Rabbits generally reproduce at a small fraction of the rate of bacteria, although exceptions exist.
Note: I'm not saying humans definitely introduced bacteria to Venus and that's the source. I'm saying if conditions are by some chance ideal, and these things are apparently thriving in the atmosphere, it shouldn't be hard to spread across the planet incredibly fast.
[1] https://www.nationalgeographic.org/article/how-european-rabb...