If you want a serious answer, it's that realities are social constructs. Someone has to have made them. In order for there to exist a universe with the answer on how to do fusion in our universe, someone has to have figured out that puzzle. It was perfectly possible for Europeans to sail to Australia, and bring existing technologies like agriculture or cars into aboriginal australian realities. And we can bring aboriginal technologies like smoking ceremonies and outback foraging into european realities. Outback foraging turned out to be pretty useful for Europeans living in Australia, and a lot of aboriginal Australians like their cars. But there are also problems, like that Europeans don't believe in the benefits given by smoking ceremonies and that agriculture requires people to buy up all the land and make traditional living nearly impossible. And since neither culture has found the technology of terrestial fusion energy, cultural exchange between these two universes is not going to provide this answer.