If Columbus had never sailed in 1492, Europeans would have learned about the New World pretty soon anyway--in 1500 Pedro Cabral accidentally encountered Brazil while sailing around Africa en route to India. And of course John Cabot discovered Newfoundland in 1497 (but he was aware of Columbus--I'm not sure if he had the idea of sailing west before he heard of Columbus).
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.