[stcredzero]

They wouldn't have had to wreck their atmosphere to that extent so quickly. What if they wrecked their atmosphere more slowly, then experienced a slow global collapse of civilization due to the climate change? There could have been a "phase change" event where they retained technology, but quickly lost the ability to engage in the magnitude of project required to save their planet. "We of Souvenusia do not trust the people of Norvenusia to act in good faith in the sun-shield project!" Venusian civilization devolves into global war. Then the climate has warmed up to the point, where their wartime activities trigger a global methane clathrate catastrophe.

It's especially not particularly plausible that it just so happens that life did it to Venus using exactly what happens to be a fashionable thing to talk about here on Earth at this exact time.

But it could make for a timely, marketable, ripping good novel!