The Death Creator achievement unlocked too early, it was reported only 5500 enemy soldiers in total, and the achievement specifically states "A weapon you created" not "Weapons you created."
The weapon that killed the 5500 enemy soldiers was TNT, and TNT was not used against any of the Gizmas, so either the Author messed up, or the translator did.
That, or - after I did some ChatGPT translating of the Raws for that achievement text, - the author was speaking in plural terms while the translator translated it in singular terms, as how the raws is written makes it possible to use in either singular or plural terms. In plural terms, it could mean that the achievement states "Weapons that you made have killed 10,000 people" or any alternative format of that.
But that means you'll have to decide whether "people" include Gizmas or not, and if it doesn't, then it's definitely an error on the Author's part, as there were only 5500 people that were killed, and that's only assuming all of them were within the blasting radius of the explosion.