Since the sun is 104 times larger than Venus, I seriously doubt that its output of radiation can be significantly influenced by Venus. I think this guy should go back to the drawing boards.
You mean, of course, the apparent size of the Sun. Which means that when transiting the Sun, Venus blocks just a little less than one percent of the Sun's energy from the Earth. That's a significant amount.
I think you are missing some orders of magnitude in your comparisons
Sun's mass is 1.989 x 10^30 kg
Sun's Diameter is 1,391,940 km
Venus' mass is 4.869 x 10^24 kg
Venus' Diameter is 12,104 km
While the sun is close to 104 times greater in Diameter(115 times to be precise) the Mass difference is what's important when dealing with gravitational effects. And frankly there is no comparison between the two.
Or, another way to look at it, Venus is not quite 1% of the Sun's disk as seen from Earth.
So, what does it mean if 1% of that disk is occluded?
Does anyone know what a 1% variation, up or down, in the Sun's energy does on Earth?
It's gotta' do something, but exactly what is it? If the Chinese have a couple of thousand years worth of records on floods, and they correlate with the Transit of Venus, quite possibly we are looking at that 1% in action.