Heat supplied at high temperature is more valuable than heat supplied at low temperature. Look up Carnot efficiency, and "Exergy". There are limits on how efficient a heat engine can be, defined by the "hot" and "cold" temperatures. A hotter "hot" side allows higher efficiency.

That sounds like room for innovation, rather than an unfair trick. If you can find ways to achieve high temperature cheaply and easily, you get better efficiency.

I think the point they're making is that 40% efficiency is amazing when you have a 1500° difference between the hot and cold side. But at 2000° the theoretical efficiency limit is higher, for all heat engines.

It's like if I compared the top speed of your Porsche on a perfectly straight road, clear weather, no wind, to the top speed of my sedan, but I've got it pointed toward a very high cliff and I'm claiming a top speed of 400 mph (straight down). It's not a fair comparison, because I am cheating by exploiting a greater 'height'.

If we actually compared the same thing, yours would still be at least 50mph faster than mine (at a slight angle from straight down)

Solar concentrators, yes. Or even cut out the steam completely and use the hot exhaust gas from the fire directly. Or combine the element into existing powerplants extracting high temperature and use the residual heat to run steam turbines.