I guess it is just that these GUI toolkit people usually are trained to
deliver good looking and pixel-precise artwork and in consequence might
have some difficulties to imagine how it can be "pretty much impossibe"
for some editor to know how many lines it is actually displaying (or
anyway that such editor would still want to use their scrollbars).
Emacs _does_ know how many lines it's displaying.
What it doesn't know is how many lines are in the entire file, or more
precisely, the "height" of the entire file as rendered, which
essentially requires rendering the _entire thing_ (it can't be
calculated from logical number of lines because it depends on the exact
characters rendered). That's a pretty nasty requirement for a non-toy
editor...