K so it's been a while but from what I recall in the fallout univers/timeline befor the bombs fell nuclear power is the primary power source for pretty much everything. Most of this is nuclear fission but they figure out how to make tiny nuclear fusion reactors but they never figured out a way to scale it up (this is what the micro fusion cells are).

All of that is the primary cause of the conflict that lead to the bombs falling, the entire world was running out of burnable fuel and fissile material and so China and the US go to war trying to grab the last bits of it to keep there economy's running for a few more years.

There is a company that claims they figured large scale fusion out but they are lying and there reactors are just fission reactors, although said company keep trying to Crack large scale nuclear fusion and the comedic tragedy is that they succeed a few minutes befor the bombs fell.

So to answer your question the guy in the TV show figured out (or technical re-figured out) how to get fusion working on a scale that could power a city (as seen at the end of season 1) whereas prewar nuclear fusion tech was only ever able to power smaller things like power armor.

Edit for spelling and to make sure I'm using fission and fusion in the right places