I tried my best to avoid an "'opinion-based' question' by sticking to the advice underneath. But why was my question still closed? My question is about specific expertise because I'll be thankful for the experts' advice here and I quoted those Cambridge math undergrads. But all these quotes are too short for me to understand. Thus, please expand?
The close reason for primarily-opinion based questions is relevant here (emphasis added).
Many good questions generate some degree of opinion based on expert experience, but answers to this question will tend to be almost entirely based on opinions, rather than facts, references, or specific expertise.
Answers to a question about learning strategy involve opinions to a much higher degree than evaluation of an integral, but they can be based on expert experience, too.