"...it is primarily an Executioner's weapon", keyword: primarily, so it is posible (but unlikely) for him to have it. But, given that it is a magical weapon, and that even forging it would require specialized tools, a forge and a large amount of steel, wich nomads don't have. This left me with 3 posible ways to adquiring it
a) they stole it from a merchant's guard, likely if it where a regular great falchion, but it's a large one
b) Found it in one of the desert ruins, wich make it much more likely given it's size and the fact that it is enchanted
c) Inherited it, it could have been passed from generation to generation, maybe it belonged to old man Fahim, and he gave it to Kashue.
I'd rather stay with c, but b is posible too.
Also, the sheat has a lock that allows it to be opened sideways, allowing him to draw the weapon at a normal speed, as getting it out in any other way would be extemely difficult.