No, he is not saying they are unbelievers and incapable of believing the gospel. In fact, if he thought they were, he would not be saying these things. The fact that they were believers (at least some of them), Paul is explaining the difference so that they can understand their practice to be falling short of God's glory. This is the reason for his explanation.
So when he says "are you not mere men," he is not claiming they are "mere men" literally. He is using this expression to mean that they are acting like mere men, acting like unbelievers. People who are self-centered, greedy, prideful, conceited, envious, and all sorts of other wickedness that comes out of the natural human heart.
So the warning that they are sinning is intensified by his explanation that spiritual people should be thinking very differently than natural people, because spiritual people have wisdom given to them from God that natural people don't have. All natural people have is what they have learned in the world through worldly pleasures and pressures, and feelings of the heart, and such things.
This should have shaken them up at least for them to realize their need to live by faith in the God who works His will through individuals. And for the true believer, it indeed does shake them up to repentance from carnal practices, because they have the Spirit of God giving them wisdom to understand it. But those without the Spirit will not listen.
So then, comes the general principle of Paul's teaching about who gets saved and who doesn't, and then how it applies to the Corinthian believers in their Christian walk.
Is this your understanding of what Paul is teaching in this passage?
TD