The article specifically mentions eye tracker users as an affected group.

Everybody who tries to actually use the scroll bar as a handle (rather than a non-interactive position indicator for use with the other scrolling methods) would have a problem, because they're not really meant to be used that way and most people don't use them that way (which is why they keep shrinking).

Unfortunately the article doesn't mention how people who don't use an eye tracker, can't use a scroll wheel/touchscreen-dragging (or adapted replacement for either!), but could use a "normal-sized" scrollbar, are actually interacting with their devices. I imagine that overlapping group to be rather small.