It's not particularly clearly worded but to me that seems to suggest that tanning increases melanin and that melanin helps prevent cancer regardless of whether your dark skin is natural or tanned.
For a fixed amount of sun exposure, spreading out the duration is better both to establish a tan and to allow more time for skin recovery. But there isn't really a case where tanning increases safety, because you get the tan via exposure.
It's like saying pilots who practice a lot are safer; for any given flight it might be true, but cumulative risk can only rise.
https://www.scientificamerican.com/article/fact-or-fiction-a...
You clearly get more than SPF 3 with a deep tan as you can spend 8 hours in direct sun without obvious problems. Without any tan you get a burn in under an hour suggesting ~SPF 10+.