I think for some people who have the proper mechanics and know how to pitch....yes they can benefit from increased power in their legs (ie squat and deadlift), core, and upper body flexibility. Keeping mind...not all pitchers rely on velocity exclusively.
My oldest son played in college. He had very good mechanics, and he put on 15-20 lbs of muscle in his legs and backside his freshman year. His top end velocity may have changed 1-2+ mph in college, but that increase could have been attributed to many things...further improved mechanics, diet and work habits in addition to squats, etc. What is interesting is that he changed from a strikeout pitcher in high school and travel to more of a multi-pitch pitcher that let hitters get themselves out in college.
So, if I had to put a myth or fact label on your premise.....I would say myth. Squats and deadlifts by themselves will make you stronger in the legs but may not translate to increased velocity unless you have a very good pitching foundation.