Here's what Mike Marshall says about why he developed his screwball...
In 1967, I thought I pitched well. In 37 games, I pitched 59 innings, walked 20, gave up 51 hits, struck out 41 for an 1.98 earned run average. However, Detroit Tiger Field Manager Mayo Smith permitted me to only pitch to right-handed batters. I had a good slider and got right-handed batters out. Unfortunately, left-handed batters hammered my slider. Therefore, during the 1967-1968 off-season, I worked on learning how to throw a screwball.
The key thing to note is that Marshall developed his screwball because LH batters were hammering his slider (because it moved down and in to them). He needed a pitch that would move down and out to LH batters.
I don't see anything inconsistent with this in what I said.