Answer

Indeed, coercion applies only to someone who believes. In the distant past, however (in the time of the Sages), the presumption was that people were believers unless proven otherwise, and therefore coercion was applied to everyone (at least so long as it had not been shown that the person was a denier). But today the situation is the opposite, and there is no such presumption. This applies not only to coercion but also to punishment for transgressions.