What I mean to say is that this issue stems from your misunderstanding of the meaning of the word “mature” and nothing else.
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.
Fascinating, it sounds like "MM" is tacit knowledge, which begs the question why it isn't explicitly taught.
In mathematics, mathematical maturity is an informal term often used to refer to the quality of having a general understanding and mastery of the way mathematicians operate and communicate. It pertains to a mixture of mathematical experience and insight that cannot be directly taught. Instead, it comes from repeated exposure to mathematical concepts. It is a gauge of mathematics students' erudition in mathematical structures and methods, and can overlap with other related concepts such as mathematical intuition and mathematical competence. The topic is occasionally also addressed in literature in its own right.
I'm not claiming that my response was entirely rational or measured. I'm not blaming the professor, and I'm not justifying my own reaction. I was just saying what my reaction was, in case it shows a pattern of student experiences that's worth addressing.
Stubbornness, tenacity and self-belief are useful skills for success - perhaps you gave up too quickly? Unfortunately, soft skills are very rarely taught well. We all rely upon our subconscious learning and a good amount of luck to gain necessary skills, and those skills are often learnt by osmosis while very young. I also think the meta-skill of being able to teach yourself soft skills is not common either and it - although we can make an effort to improve that meta-skill even though it is somewhat self-referential.