To be fair I read it as meaning we would be incapable of discovering them, rather than that they didn't exist. But yes, taken as a whole the post is rather eccentric.
I wasn't around at the time, but from the science fiction of the era, it looks like nobody really thought about how to detect exoplanets from Earth, because everyone assumed the curve of technological progress in propulsion would keep going and you'd just send scout ships to other stars to survey them up close.