"1.3 If the position is such that neither player can possibly checkmate, the game is drawn."
That seems to cover insufficient material.
Yep, also 5.2.b, 6.9. But it's not defined as "insufficient material", in-fact material is not mentioned anywhere. Because, in blocked positions you can have a lot of material, and still you would not be able to checkmate.
That's why I said "so called" as the FIDE rule is more generic and it covers a situation where a casual definition as "insufficient material" would fail.
So, no one has any evidence, except for empirical one.
You are clearly incapable of understanding. Peace on you too.