See D7351 for some more info.
But discussion there is starting to go beyond concerns of just that patch. Even if that patch is reverted we're still left with a bug.
If I delete my ~/.elementary directory I'm subjected to "thumb scrolling" by default, which does ludicrous things. With that patch reverted this doesn't happen, but that's only because we don't fall back to the hard coded defaults.
I personally like meaningful hard coded value. The main question would be why do we have weird hard coded value? Is it because nobody is testing them? Or is it a choice?
My guess on thumbscroll being hardcoded to enabled is that someone working on elm for touchscreen devices/or a patch from Tizen who works on it with touchscreen devices, or something similar set thumbscroll as enabled somewhere along the way in the hardcoded default without considering the desktop use of elm. Either that or the values just have the hardcoded values but because they weren't used, they weren't tested, and no one cared.