1) Comparing scientists and "the general public" is quite misleading. A better comparison would be of creative scientists to intelligent people from well-to-do backgrounds who lack scientific creativity. I suspect that comparison would be much less dramatic.

Irrelevant for the proposed use.

2) He uses "sales of one of my books" as a proxy for "adults interested in science fiction". The hubris is staggering. Yes, Asimov was a popular science fiction writer. But there's still no support for the assertion that on average every serious fan of science fiction will own this particular unidentified book.

Considering only Asimov's sales makes the 1-in-450 or whatever too small, and exaggerates the power of the test. But it does not reduce it to an inverse correlation or even just uncorrelated.

Nor may it be too much of an exaggeration. Asimov was very popular - he was the iconic SF writer, bigger than Heinlein. My father was not an Asimov fan, yet he still had like a dozen of his books in his SF collection. I'm not a big fan of him either (his fiction writing is terrible as literature), yet I read them all because to be SF-literate, I had to know Asimov's ideas like the Laws of Robotics.

3) These two points exacerbate one another. Note the shifting of goal posts. "Half of scientists have some interest in science fiction" versus "only a small fraction of adults have bought my book."

A minimal goalshift per #2; Asimov does not have detailed surveys available about what SF, exactly, the creative scientists were reading. (I am surprised you object to the use of Asimov and not the list of prescribed authors he gives like Budrys or Heinlein.) As a matter of fact, I do not think such information is available now, 50 years later.