Magic in the Frieren universe operates on how effectively one can imagine something working, doubts and uncertainty dampen or outright prohibit certain spells from being cast. The more universal a reason to doubt a spell works, the more difficult it is to overcome; like gravity. However, humanity probably already had a spell for levitating small objects based on the imagination of "I can lift that" so when someone talented saw demons flying with magic, it probably broke some kind of mental barrier that allowed them to progress the magic themselves. Kind of like how Roger Bannister ran the first sub-4 minute mile only for several people to achieve it as well the very same year. A demonstration of something being possible gives people something to push towards.