"mathematical maturity" would be a combination of "not smart enough" and "haven't studied enough". Highly talented people can reach high levels of maturity very quickly, people who are just a bit above average can reach a reasonable maturity through hard study, and below average talent means it is very unlikely that the person will be able to comprehend university level theoretical math.

A teacher who doesn't know you will not know if you're not smart enough or just didnt study hard enough, and probably doesn't care much, especially for an undergrad. For a phd student, they may care if they think they've identified a "diamond in the rough" that can be turned into a gem with some work, but that is quite a bit beyond basic linear algebra.