Because of what it means to believe and to have an opinion.Why would he need to hear the phrase "god" for the first time to have no belief about it, and hence no opinion? Didn't he already have no belief about it and hence no opinion before he heard the phrase "god"?
The world, for each of us, is composed of all the thoughts and ideas we have about the world. Thoughts and ideas we don't have cannot help compose that world. The ones we do have, we can put into words.
Belief about something you've never heard of is not a belief. It doesn't exist.
To say I have "no belief about something I don't know and never heard of and doesn't exist," is to say an awful lot about it. It contradicts that you have nothing to believe in. You have a very large lot of words there representing what you say you don't believe in.
Or you could just say nothing.
But to say nothing is not to be the atheist. The atheist is the one who doesn't believe in god.
To say nothing is to be the Buddha.