Of course Asimov oversimplifies things. He's also playing to his audience and being sarcastic and humorous. He's being snarky, and when snarkiness is the goal, accuracy suffers.
We don't even know if this professor actually existed, or whether he/she was invented by Asimov for the sake of his essay.
I think the message is valuable though, even if there are nuances and exceptions to it. A "rule of thumb" if you will.
The idea Asimov was fighting against was this "new age" belief that nothing can be known, everything is equally wrong, everything is "opinion" and yours is as valid as mine, etc etc.
PS: a tangent, aren't things like string theory pretty controversial (in the sense of "is this real or fantasy") even within the Physics community?
Yeah that's valid. I do find philosophical discussions about the nature of knowledge fascinating but it's often used as a bludgeon to assert one's opinion as being equal to fact.
Take stomach ulcers for example - before modern medicine people didn't know their cause, but were able to somewhat treat them using bismuth compounds. Doctors and scientists refined their theories over the years based on the concept that they are caused by stress, spicy food, and excess stomach acid, developing various cures, until Barry James Marshall discovered that they were actually caused by H. pylori. Before this discovery it would seem reasonable to think we were "less wrong", that because our theories became more refined that we were closer to the truth.
As theories of the universe become more uh, theoretical, we cannot accurately measure and gain a sense of if we are moving closer or farther from the truth. Perhaps this was the point the professor was getting at, or maybe it's just some postmodern "knowledge doesn't exist" thing - we can't really tell because Asimov only paraphrases the professor. But I increasingly get the idea that we don't actually know much of anything about the universe based on the seemingly insane theories coming out (eg string theory, infinite worlds).