These things are new (to me), so I might well not fully understand them..
But, it seems like they work so well, because of the vacuum around them, inside the 'evacuated Pyrex tube'.
That vacuum keeps the heat from being
re-radiated..
You couldn't pull much of a vacuum on a flat-plate, before it collapsed in on itself.
That impressive video (above) seems to show the copper just keeps on
accumulating heat and not re-radiating much of it. (Just on the bare end).
Getting up to over 212F seems surprising, since the light capture area is so small.. But, if no heat is escaping..
It might be an interesting experiment, to suspend a solid copper tube inside a clear vacuum bottle.
Sit it out in the sun a few hours and see of it starts glowing cherry red..?..
If no heat could escape, then maybe the copper might just want to melt into a puddle?? LOL!
There must be some frequencies of light (IR?) that will be re-radiated,
(losses) otherwise it would get infinitely hot.. All gain: No loss..