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...