Venus has a hot patch

According to this article and image, geo-astronomers looking at data from the Magellan mission have spotted an apparent recent lava flow, recent because it's warmer than the adjacent surface.

Cooling lava flow spotted on Venus

There's something I don't get in this article, even though I think the observation is valid. Here's what I don't get:

"The flow must have been at least 15 years old when detected by Magellan, she says, because the Pioneer Venus orbiter photographed it in 1978."

As far as I remember, Pioneer Venus only photographed cloud tops. How did it see a lava flow on the surface? Reading around indicates that somehow Pioneer Venus saw it, and so I'll have to figure out how later.