I think this opinion is much misunderstood.
It should read: people who are not interested in what they are doing are unlikely to ever become very good at it.
People who are interested in a topic will have their brains work overtime at different times of day, will be picking up information even if they are not specifically looking for it, will be trying to understand stuff they are not specifically required to, and so on.
As pointed by some people (like David Allen) our brains have something called Reticular Activation System which, if programmed, brings to our conscious attention when something interesting happens. And then we will also have easier time keeping focus.
People who like coding will probably not see it as only work but also choose to do it in their own time and that usually results in private projects.
I do electronics at home and this usually includes some kind of microcontroller and coding, just because I like it. But I don't have public repositories and I don't do open source projects because I do my own fun projects at work and get directly rewarded for it.