I'm pretty sure that data came first. Perhaps it was RADAR data from Earth. The temperature was measured.
It was a bit of a shock that Earth's twin sister is uninhabitable.
But is it? It turns out that if you go high enough in the atmosphere, so that the pressure is similar to the Earth's surface, it's also a reasonable temperature. So perhaps we could colonize Venus right now - in air ships. Big balloons. Of course, there's still absurd winds, and sulphuric acid, and other stuff to deal with. I mean, we don't colonize Earth with air ships... even though we could.