Sure, I believe you can rigorously find brain patterns that are correlated with understanding. The obvious, "correlation is not causation" applies, however.
The problem is such pattern aren't any guarantee of understanding. Just "feeling confident" seems like such an easily correlated thing. More people who learn a subject sufficiently to do well on a test might "feel confident" but certain kinds of people might just feel confident without that actual learning.
Causation also implies well sequential causality in some way and reproducibility. A subtle but crucial difference.
If most people on the battlefield are covered in their own blood will soon die that doesn't mean gathering a gallon of a person's own blood through several donations and then throwing it at them at a Gettysburg musuem will kill them.
As Feynman discussed in "The Character of Physical Law", there is more to it than that, but I think one can say that the empirical evidence underpinning all science can be characterized as correlation that passes various statistical tests.
The problem is such pattern aren't any guarantee of understanding. Just "feeling confident" seems like such an easily correlated thing. More people who learn a subject sufficiently to do well on a test might "feel confident" but certain kinds of people might just feel confident without that actual learning.