We saw how big and intense his katons were in war arc. Had his wood not been resistant, it wouldve been swallowed up whole like all the water jutsu evaporized, even those of mei terumi.
Did we even see the full fight between Hashirama and Madara to even say that? Also, those big and intense Katons were stopped twice, by some random shinobis and Mei.
Again, I can show you how Mokuton was burned by Katon, but you cannot show me otherwise, and at best you're guessing what happened in a fight we didn't see.