Waterlogged timber. Look at the bottom, it's rotting.
So many people pretending they know what the fuck they're talking about, and make a bunch of claims, but by all measures: a log of this size, this straight grain timber, this old, being hit with this maul, in that spot, by this man, using his technique, it should have split. The problem was, it's wet timber.
I've come across logs exactly like this; it's like bouncing a sledgehammer off a tractor tyre.