If you're talking about page 5 of the 2015 edition (p.7 of the 2003), it's not from the Diarios translated and digested by Bartolomé de las Casas in the 1530s, but the latter's own multi-volume History of the Indies drawn from his observations and wider reading that included it. Zinn makes this clear, but if you miss just a few words it's easy to misattribute it, and the lack of footnotes doesn't help.

Finding the original passage in question has been elusive because my Spanish is abysmal, although it does appear in English in the 1971 (condensed?) translation. As to whether it's a true report, we have only de las Casas to go by on this as with so many other things. So de las Casas did write the passage in question, but whether his understanding was deep enough to make the statements, and to what degree his knowledge reflected the dual biases of individual and memory (he wrote this decades after the actual events of contact), is almost impossible to verify. However, the focus on sexual license and differences in various related practices that would appear fantastic or repugnant was a common feature of moralistic travel writing and can be found in accounts of other areas undergoing colonization as well. That makes figuring out the reality underlying such statements all the more difficult if we have only one or two sources and nothing from the people whom those sources discuss.