Yes, he is.It is an expectation that you and those people are putting on Van der Poel because he has never said that he wants to be the best classics rider of his era, he has already stated his goals, having the record for most Flanders wins and winning the MTB Worlds, its as simple as that. Now contrast that to Pogacar that has said that he wants to become the best rider ever.
As I said, you are entitled to think that Pogacar is a better classics rider than Van der Poel but you will have to live with the fact that the Dutchman is also one of the best classics riders ever.
But he is not the best classics rider of his generation.