This stat is not capped, but doing more than 2.14 billion damage in one hit will crash the game.
Throw damage has different components to it, which can be seen in-game by mousing over the throw damage stat in the journal.
Base is the base damage that is done by the peasant throwing their pickaxe. This value will have a small variance added to it, but if there are no modifiers or crit chance, then this is the damage that is done to the enemy.
Base modifier is an additive modification to the base amount that will increase or decrease the base value.
Base multiplier is a percentage multiplication to the base value that will increase or decrease the base value by that percentage. For example, if it's a 10% multiplier, that means it will multiply the base value (after the Base modifier) by 1.1.
Bonus is an additional boost after the calculations, which is additive.
The rest of the stats in this dialogue have their own page.