You are right, it is real, and quite interesting. The reason it ran is that about 2 billion years ago, the ratio of U235 to U238 was quite different - there was a lot more U235 around due to the difference in half-lives between these two isotopes. The result was that you could basically run a "light water" reactor from natural uranium, which you can't do today. Now that U235 has decayed away we require enrichment for regular water to work in a reactor, or we require "heavy" water, such as in a CANDU reactor to make our current natural uranium work. The Youtuber "SciShow" did a great ~4 min vid on this topic as well.