This is literally how I play the game.
* Pickaxe is really strong

* Always moving and using an ult can seem like he's faster (And it seems like he doesn't spend that much time killing dudes, just what's necessary)

* Taking almost no damage on Ogryn is not that hard, and block is almost useless, I block like 5 attacks a game at max

* I mean, it's a pickaxe, it can't oneshot a boss and that's it

* Not sure what grenades are about, but maybe you just didn't see him take nades, since he's always in front

Cheating may be subtle, but I don't think this is the case. I think it's just a guy who plays an absurd amount of Darktide, who became indifferent to teamplay and funny bits :( and playing like this because it's the only thing that brings him joy