Probably a semantic difference that I don't equate proof with understanding.
I would call that a colloquial definition of understanding. Very different from mathematical understanding.
If you ask a mathematician what you’d need to know to understand Fermat’s last theorem, he won’t say “high school pre-algebra.” That’s only enough for you to understand the basic statement of the theorem. It doesn’t get you to the why. To understand the problem involves a deep dive into both algebraic number theory and analytic number theory.
On the contrary, I use "understand" quite precisely. It's interesting to study what practitioners of various disciplines mean by "I understand."
For example, most mathematicians would say they understand large numbers of theorems that they cannot prove off the top of their heads. On the other hand they probably have what Borovik in "Mathematics under the microscope" called a "recovery procedure." That is, a set of constraints or path that reproduces the result. They know they can reconstruct the proof if they need to from the various gambits and skills they keep polished.
Also, the proof via the normal distribution being an attractive fixpoint of convolution is fine, but it only works on a particular subset of functions. We know the theorem applies beyond that subset, and there's a cottage industry of extending it in bits and pieces and calculating better convergence bounds. There is no proof available today that says central limit theorem applies iff conditions x, y, and z. So in this case the proof really can't be said to be understanding.
Now, that's well and good for probability alone, which is a field of mathematics. Statistics isn't a subfield of math, or is a subfield of math the way physics is. For a statistician, understanding the central limit theorem is much more about knowing what kind of observations it is reasonable to expect it to approximately apply to, what kind of tests rely on it and which don't, how to check if it applies in a rigorous way, what kind of visualizations and exploratory data analysis is enabled if it does, how the normal distribution and convergence to it fits into a whole family of distributions and features thereof...
