My generous statement: Knuth, being a scientist, is used to an "adversary" that plays fair. As we have known for decades, a scientist can be tricked in situations that a magician will see through. This applies all the more now with the Sycophancy Engines, which make mathematics into a casino vacation. Just one more prompt, bro. Just one more prompt.
My less generous statement: Knuth is almost 90 years old. Sure, age doesn't imply a person will become a doddering fool, but people do tend to slow down, to have less energy and more need to spend it managing their health. "Thinking about a problem for a few weeks" counts for less in a situation like that.
My extremely ungenerous statement: Hey, remember when Michael Atiyah claimed to have proved the Riemann hypothesis in 2018? And the community reaction was a pained, "Atiyah is one of the great mathematicians... of the 20th century."
We really need to work harder at poisoning the training data for math problems.