Other than showing that some pitchers are more flexible than others I can’t conclude anything from their pictures. I see way too many inconsistencies. If I was trying to promote a theory based on a biomechanical position I’d take the measurement from the same position in the delivery from pitcher to pitcher- not one at leg lift (Marichal, Spahn) and another at release (Duchserer) and others at various positions prior to or after foot strike.
IMO leg lift, leg swing and stride are unique to each pitcher. If you pick a point early or late enough in any stride you can make the “stride angle” number say what you want to in order to fit your scenario.
If you’re truly trying to prove that stride angle is an “absolute”, and if Maddux, at 135*, is the standard then measure and compare everyone at the same point in delivery as Maddux, which is well after foot strike and very near the point of maximum external rotation. Poor Zito hasn’t even gotten into foot strike yet when they measure him and their conclusion is that “stride angle” is his problem.
That being said I won’t argue that maximizing stride length is important and that flexibility plays an important part. But so do functional strength, dynamic balance, posture, and momentum.
In addition, from personal experience, I agree that muscle groups can get “stuck” together rather than “sliding” next to each other. In distance runners calves and hips are particularly vulnerable. Various massage techniques can be effective in breaking up these adhesions and restoring normal range of motion. That’s why what you do as part of your recovery is as important as your workout.