That code is to implement breath effect. Has nothing to do with mouse acceleration.
in one quick search in the source files i can see that function being called more than 50 times.
somewhere there you guys are ruining the mouse input.
open the game, set mouse filter off, m_customaccel 0, m_filter 0, m_mousespeed 1
and look at a wall, move your mouse from the far left of your mouse pad to the far right,
then go back to the same spot you started, and do it fast, you will see you can turn almost double the
distance while going fast.
its on, and its forced, and for a engine that has such a good mouse feel for all other games its unfortunate that
some kind of mouse filter / acceleration is forced.