The Year of New Editions means a lot of work for a lot for a lot of game designers. So in the initial release of those new editions, there might be parts of the game that had been around, but weren't updated right away. This means the games get to you quicker, but then some catch-up has to be played. Well, the folks over at One Page Rules are working to bring everything possible to the new edition of One Page 40k. That includes Genestealer Cults and Space Marine chapters.
This gives Tyranid players a bunch of new options to play their army. You don't just have to be the "wave of bugs" that Tyranids can sometimes be. As for Space Marine players, no need to just play "vanilla" marines anymore. Dark Angels, Blood Angels, Space Wolves, you can now differentiate yourself from your other 3+ armor save brethren.