Fun read! It still surprises me no predator has been able to somehow take advantage of this evolutionary gap. You'd think one of the predators mentioned in the article would evolve a different way to the detect them.
Then again, something similar probably happened many times in evolutionary history, and the victim species died out as a result. So if one of those predators would exist, we wouldn't have sloths. I guess this leaves them vulnerable to invasive species?