Fusion products have particle energy on the order of 10s to 100s of MeV. Thermal machines tend to melt or evaporate when their average particle energy gets on the order of 100s of meV.

The increase in entropy on that temperature conversion alone is absurdly wasteful.

My understanding is that they are talking about bulk heating of a metal carrier, probably by a tens to low hundreds of degrees for power production. Not every particle in the medium is at Mev, otherwise it would be trillions of degrees kelvin.

In direct conversion, all of the particles used on the conversion have MeV energies. It is exactly that bulk heating that is the problem.

When you get a very low entropy source and your first step on using it consist on increasing the entropy 1000000000 times, you lose a lot of flexibility and efficiency.

Nuclear fusion is pretty much the exact opposite of a low entropy source. In any case, thermal power plants (gas, coal or nuclear) reach up to 50% efficiency with modern gas or steam turbines.