I am not sure where you grabbed that, but if anything, there was a tendency for the puzzles to get easier over time. Many of the hardest puzzles came from a time before they were publishing hint books.
I had found that the hint books didn’t really help with the most difficult puzzles. For me, they usually involved maneuvering the character in a certain sequence. For instance, in one of the King’s Quest’s the character was inside a whale’s mouth and had to climb up the tongue to escape. I had a LOT of trouble with that. In Space Quest 4, the character had to jump a certain way on a trampoline. The hint books didn’t help at all with these.