alt Hacker NewsLiterally 100% of that heat travels from the 1000000C stuff to the environment throught that vacuum. Vacuum doesn't just remove energy.
If you use a steam engines it doesn't matter if your source of heat is 900C or 1000000C, all heat will be captured, and 40-60% will be turned into electricity.
What you said there is all true, but largely because you didn't mention efficiency. If your heat source is a lot hotter than the steam you make, you do lose a lot of efficiency. If you had a million degree heat source, you could have many steps extracting huge amounts of power before your "waste" heat gets down to 1000C and is used to boil water.
The part about bad conduction being a problem is nonsense. The "lucky to get 1% efficiency" is not nonsense.