Paul Samuelson is an over-rated, ignorant Marxist with a limited understanding of real economics. You could memorize every word of one of his text books and still know very little about economics. Not understanding economics as it's taught in one of his text books is a good thing in the long run.
Paul Samuelson is now best remembered as praising the Soviet economy in the 1989 edition of his eternal doorstop.
It was P.J. O’Rourke who memorably reviewed the textbook in one of the chapters of his economics-centered work “Eat the Rich”.