i have a good feeling that this was a rage thread. If he had 500 ammo he probably wasnt spamming mines like other 1v1 players.
I doubt you will even need 500+ ammo. Unless it is fixed, the scavenger doctrinal artillery is still self-repeating without costing ammo after the first strike. Unless you fully repair your base in between, the repeating strikes will clear out the US base.
In any case, even if it was a rage thread, it would totally legit. The scavenger doctrine doesn't have other ammo consuming abilities, and the surprise grenades are basically free. It's not that hard to save up lots of ammo in the late game with this doctrine because there aren't that many things to spend the ammo on at this stage of the game.
For the sake of balance, this arty should not be usable on base sectors.