That "if" makes me understand that spending all day in the sun is not something you normally do. I've been told by others who live in Texas that you can build up tolerance and spend all day in the sun.

Naturally darker skin does not automatically provide sunburn protection. It is spending time in the sun that adds the missing part that provide that protection. I don't know how this differs from dark skin, but I've seen it myself.

Note that my claims are only about sunburn. The article/topic is skin cancer which may well have different factors from sunburn.