That has to be one of the stupidest arguments I have ever heard. That's like saying that since I'm not married to every woman I must be single even though I am married to one woman.
Now doesn't that sound stupid to you?
What you said certainly sounds stupid.
Let’s extrapolate the examples.
10,000 gods total.
One man believes only one of those gods exist. He believes the other 9,999 gods are fictional.
10,000 women live in one town.
One man married one of the women. He believes the other 9,999 women do not exist.
This argument fails mathematically.