The people had very little machinery now. I had been struck with this over and over again, though I had not been more than four-and-twenty hours in the country. They were about as far advanced as Europeans of the twelfth or thirteenth century; certainly not more so. And yet they must have had at one time the fullest knowledge of our own most recent inventions. How could it have happened that having been once so far in advance they were now as much behind us? It was evident that it was not from ignorance. They knew my watch as a watch when they saw it; and the care with which the broken machines were preserved and ticketed, proved that they had not lost the recollection of their former civilization. The more I thought, the less I could understand it; but at last I concluded that they must have worked out their mines of coal and iron, till either none were left, or so few, that the use of these metals was restricted to the very highest nobility. This was the only solution I could think of...

What is the importance of the narrator's statement that "I concluded that they must have worked out their mines of coal and iron"?