The author spoke of native Americans, not imported Spanish slaves, but let’s run with it.
You may be right about eventual emancipation chances, but in actual brutality the French in Hispaniola probably led the pack, at least if you agree death rate of newly imported slaves is an appropriate metric.
ALL the Latin American and Caribbean slave countries had a negative population growth rate among the slaves. Populations of workers were kept up only with constant importation.
OTOH, the English colonies on the mainland (what’s now the USA) are the ONLY slave society in history of which I’m aware where the slaves had a massive natural population growth rate. Their birth rate was fully equal to that of the whites in the colonies and later states.
It is not unreasonable to assume that cruelty and brutality shortens lives and makes reproduction and survival of children to adulthood less likely, while reasonably decent treatment encourages population growth.
I was thinking of something I had read some time ago that discussed the status of slaves in the different legal systems. How it worked out in practice would depend on various things—being a slave on one of the sugar islands often meant a short life expectancy because the slaves were worked so hard (since replacements were cheap). In the English colonies it probably made a lot of difference if you were a household servant in New England or a plantation field hand in Virginia or South Carolina. And the white indentured servants didn’t have a good time of it either—many of them died before their period of service was up.