The laws of kinnim, which are the bird offerings, and the laws of niddah—when the cycle actually starts, when it ends, and when a person is a niddah or a zavah, which is a different type of a emission that a woman has—are what the Mishnah is calling essential laws, areas of study that are essential and fundamental. Astronomy and numerology are the desserts of wisdom. What is happening here, really, is that, as the Kesef Mishneh explains, the Mishnah is referring to kinnim and niddah laws.
Kinnim and niddah laws aren't that popular. A bird offering has very complicated laws. What happens if birds get switched with each other? What if a bird was designated for one type of korban (offering), and then you want to use it for a different type of korban? These are laws that a person would not think are that valuable, and niddah, a woman's menstrual cycle, is not that pleasant of a topic to be talking about. But compared to tekufot and gematriot, which are considered to be astronomy and numerology, which is like math, those are considered to be important topics to understand the workings of the world, science, and everything that comes with it, even though they're looked at as so valuable. We say that niddah and the kinnim topic are considered to be gufei halachot (the actual laws of the Torah). They're much more important. Everything else is secondary to them.
When you're talking about the reward that Hashem gives, Hashem gives a reward for studying the Torah, for studying gufei halachot, the actual laws of the Torah, even on such abstract topics like kinnim or niddah. Even when comparing that to astronomy and numerology, which are like math, those are considered to be foundations and fundamentals. But they're only considered to be the dessert of wisdom, not wisdom themselves. The real wisdom that we have is the Torah.
Others explain that when we're calling numerology, which is really gematriot also, where you're adding things together and each letter of the Hebrew alphabet equals a different calculation, we're calling that dessert because it's sweet. But it doesn't give actual nourishment. It's after the main meal that you just had; a person wants to give himself a little pleasure, he'll have a little dessert. Same thing with the gematriot. Dessert gives pleasure to a person who eats it, but it's not considered to be fundamental. It's not considered to be an idea that a person really, really needs. Same thing over here. A person who learned the actual Torah—that's considered to be the main meal. Niddah and kinnim are the main meal. Everything else is considered to be batel, nullified to that.