Since the book is titled "Critical Thinking," they probably mean that this isn't a good argument to believe God exists.
It does accurately, factually explain why the person raised a Baptist believes God exists. But it does not justify this belief; it explains why the belief is held without supporting the belief itself.
Formally, you are right that it is technically an argument. A formal argument doesn't need two premises, or even any premises, and it may be fallacious. But it looks like this book uses a different definition of "argument":
Time to look more closely at arguments—the kind that actually show something (unlike the red herrings and emotional appeals and other fallacies we are going to be talking about in a moment).
So this book, at least in that section, reserves the word "argument" for arguments whose premises do support the conclusion.