Xur is no longer random. Bungie does now have the ability to override what he sells. And also, the gjallahorn was removed from his loot table soon after he sold it the first time.
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They have always been able to do that. They made him. Duh. They choose to keep him random though. Gjallarhorn was never removed from his loot table. First of all, there was and still is no need to. Second, I can guarantee that you will not find a single official shred of a confirmation on this.