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)
