I mostly derived it from other worlds - mainly Chash's. She doesn't say much on the impregnating abilities of female alphas, but she does say that male omegas can't get anyone pregnant. Since the knotting world is a heavily mpreg set of fics, and... probably due to influence from other weird worlds, I figured it would be internally consistent to have it work like this. Other fics have slightly different rules, and the very small set of fics that contain female alphas occasionally do even stranger things to the biology. This one seemed sensible to me.