I think the X-1 is still in the camp of furthering advanced expensive technology and pushing the boundary of what is possible, compared to SpaceX and their efforts to make space transport practical. In support of the author's theory think of the significance of the Ford Model T compared to whatever the first impractical car was.

But I don't think it's possible to judge historical significance of anything with so little hindsight available. Give it another few decades and then let's talk about what the most significant spaceflight development was.