The problem is that even the theory behind that is wrong. In fission with a thorium breeder reactor you can in theory have zero cost fuel, even natural uranium would be even zero cost if you went with a fast breeder. With fusion you will need to continually breed the fuel.
In addition, the materials you need for a nuclear reactor are pretty simply and can be done with 1960s technology, while the materials for a fusion reactor are literally unknown.
So all the assumptions about fusion are just wrong, the increase in energy density of fusion over fission is pointless, unless maybe we want to travel do another star system.
Fusion is unlikely to be cheaper anytime soon, even if somebody could build a plant that makes positive energy.