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The closer you are to the equator, the less is the difference between day time and night time over the year. In spring and summer, where the days are long, I also need very little sleep. 5-6 hours. Absolutely within the range of above mentioned societies. In autumn and winter sometimes even 9hrs are not enough for me. Fortunately I can 'extend' the days with certain light sources.

And that is exactly what the article says:

Hunter-gatherers sleep an hour more in the winter than they do in the summer.

Even though:

It appears that their sleep time may have more to do with temperature than with light.

Maybe. The temperature in my bedroom is almost constant over the year... so no own experience here.

But I really would like to know, if there is a correlation of distance from the equator and sleep time.
Namibia is farthest away from the equator. Could it be that there the sleeping time is more in the 7.1 hour range? While in Tanzania, which is closest to the equator, the time is more in the 5.7 hour range?