Blow just says "then the roman empire fell and that knowledge was lost" when discussing the Lycurgus Cup. He just states this. No citation or investigation. This is the classic oversimplification of both technological record keeping and the nature of the collapse of rome. There is no clear evidence that the "fall of rome" (which means about a dozen different things that happened over 1000 years) is responsible for the mechanism for creating such glass being unrecorded.

He makes the same claim about alchemists fire. Again, ill supported.

There are a tremendous number of reasons why ancient technology might not have surviving records to today. Amateur historians love to just assume this is due to "civilizational collapse."

He also seemingly inverts things. Is civilizational collapse the cause of loss of technology? Or is it the opposite? Does technological de-sophistication cause civilizational collapse? He seems to argue both in his talk.