I'll rephrase the argument to expose the reasoning flaw.
Say, I show you an iPhone and say:
"Here is an iPhone." This is a cold fact.
Next premise:
"All iPhones are made by Apple company".
You and I may both agree that this is true based on information otherwise available to both of us. This is the key.
Conclusion: "Apple company exists."
Now we replace "Apple" with "God". The argument would go as follows:
"At least, one iPhone exists." (still a cold fact).
Next, premise:
"All iPhones are created by God." There is no agreement or evidence that this is true.
Therefore, conclusion "God exists" cannot be made from the two premises.
In a nut shell, one premise is a fact, another premise must be either a fact or assumed to be true to make this argument valid. So, although this is a valid argument between believers, it does not work for atheists.