If Pluto is about 30 AU from the Sun at its closest, it receives 1/30*30 = 1/900th the amount of heat and light as Earth. Wikipedia seems to confirm this by saying that Earth receives about 1.4 KW per square meter, Neptune gets about 1.5 W per square meter. At its closest Pluto is slightly closer than Neptune.
So, when Weinbaum describes Pluto and claims that "high noon was hardly brighter than full moonlight on Earth," I think he's got it badly wrong. The trouble is that he hasn't got it wrong the way I initially thought - Wikipedia says that the full moon is about 500,000 times fainter than the sun, so noon on Pluto is about 600 times brighter than the full moon on Earth. It's counter-intuitive, but I think it's right. It's still dim light, a few hundred Lux compared to 20,000+ for a bright day on Earth, but we're looking at the same order of magnitude as an overcast day or an office with reasonably good lighting.
Now I have to explain this away as an illusion caused by the tiny size of the sun in the sky (or something).