Yes you have a good point and people should know this so good thread.
Ian doesn't specifically say it but you can see that ferrer's feet are pointing in different directions upon a move. (his pivot). opening one foot this way or having feet not neutral will facilitate hip movement which helps in balance and quickness. Ian mentioned the neutral feet on the split step because feet are supposed to be neutral on the split step while in the air. the the feet changing direction happens on the way down with the landing of the other foot. same principal in the gravity / drop steps that you can see in the dr serve videos. Also on groundstrokes. When the feet point in different directions the hip will be more involved.
Almost all top players do this on their fh stroke but for some reason nadal doesn't a lot of times. He hits his fh while his feet are neutral and pointing in the same direction