I think you’re confusing proving with understanding. The two are almost orthogonal.
Under your thesis one cannot "understand" anything physical (as in physics, chemistry, etc) as the underlying cannot be "proven", at all, by definition.
But usually true understanding and formal proofs go hand in hand.
P.S: Though as a disclaimer, I don't know maths beyond the undergraduate level, and I'm sure there must be very complicated concepts that mathematicians don't understand, but can reason formally about, as well as concepts that mathematicians feel that they intuitively understand but can't quite prove.
Additionaly, there's von Neuman saying "Young man, in mathematics you don't understand things. You just get used to them"