Seconding both points. I'm not one of those cases, as I could already sing decently, but I've seen people go from "terrible" to singing professionally.

I also agree that the linked page isn't useful, it's more of a glossary than anything, but then again, I'm not convinced that a distinction between head voice and chest voice actually exists. I've never been able to tell any qualitative difference, as opposed to, for example, falsetto, and the community can't really agree on whether they actually are a thing or not.