That passage reminded me of a remark by the 2002 French Fields Medalist Laurent Lafforgue:
"My specificity today among French mathematicians is not to know more mathematics than others, I can even admit that I know less mathematics than most French and foreign mathematicians. It's not about being 'great' either, I'm not great at all, on the contrary I have a rather slow mind. When I listen to a seminar, I'm probably one of those who understand the least what is being said: an idea in an hour, for me, that's already a lot. No, the only explanation for the successes that I have been able to obtain in mathematics is my being imbued by the reading of the great classics."
https://www.biennale-lf.org/b22/index.php?p=https://www.bien...
As for your experience, you are indeed very far from alone.