This is true! This does not mean that your full swing and longer clubs will be neglected. Just by working on pitch shots from 20-80 yards you are giving yourself a good feel for the impact position and how you should strike the ball with your full swing. And, not only do you feel the impact position you can improve your tempo with these shorter shots because you’re not trying to hit the ball so hard. The longer your pitch or shot gets you just need to take the club further back while maintaining the same tempo with all your shots.
Take most of your time working on all the different shots inside 120 yards but know it’s all helping your long game too.