The principle is not a mere generalization. It’s a conditional rule that doesn’t have exceptions. If an activity that promotes a child’s intellectual development detracts from social development, then it’s not healthy for children to engage in it.

The conclusion is that Megan’s reading isn’t healthy. Whether there are benefits to reading that outweigh health effects has no impact on whether her reading isn’t healthy.

The principle is about what’s unhealthy. It tells us that if the activity that promotes intellectual development detracts from social development, then it’s not healthy for children to engage in it. So it’s not “solely” a claim about what is healthy.

The author assumes that Megan’s reading detracts from her social development because it reduces the time she spends interacting with others. This overlooks the possibility that reducing time interacting with others might not detract from her social development.

A necessary condition for an activity’s being healthy is that it doesn’t detract from social development. The author does not argue that an activity is healthy because it doesn’t detract from social development.