I don't know the answer to your question but I wouldn't compare modern sloths to ancient ones. They're each adapted for completely different environments.

That's exactly the point of the question, though: when/how did sloths transition from "ancient" to "modern"? If ancient sloths were massive and formidable prey, as per the article, what happened? Were there always smaller and/or slower sloth varieties, and we just killed all the giant ones?

EDIT: As always, Wikipedia comes through [1]. There were species of different sizes, with tree sloths always being smaller than ground sloths [2], the giant of the article

[1] https://en.wikipedia.org/wiki/Sloth#Evolution [2] https://en.wikipedia.org/wiki/Ground_sloth