Read the article. He is talking about 600C heat. There aren't any heat pumps that a) give you significant gain when you already have ΔT that large (because 900

Read the article. He is talking about 600C heat. There aren't any heat pumps that a) give you significant gain when you already have ΔT that large (because 900K / 300K is a big ratio) b) have a working fluid that avoids decomposition at 900K