Fusion is different in that the math scales well with the size of the fusion plant you create (and also the power of the magnets you have access to). If it's technologically feasible to create a large enough fusion plant, it starts to be able to create ludicrous quantities of electricity compared to things like geothermal that have much harder limits.

Of course, "technologically feasible" is doing a lot of heavy lifting, but it is in the realm of theoretical possibility for a large scale fusion plant to be cost effective.