He assumed that if he personally couldn't solve a problem in a few weeks of hard work, then nobody coudl. This is kind of naive or possibly arrogant. It also sounds weak to me, coming from academic research, in which people work very hard on problems for years, and sometimes even succeed at solving those problems (famous example: Fermat's Last Theorem).
He explicitly says in the post that he's not claiming the problem is unsolvable. If I read it right, he even thinks it is possible that he could solve it, but he doesn't want to bet a year of his life on it.
