It's true in a way but just pushes things back a level of causality: if we train more people as aerospace engineers, then we will have fewer trained people in other specialties. We have a finite supply of enthusiastic, smart young people to drive change. The best answer of course is "do both" and make sure more of our young people are happy, enthusiastic, and able to access necessary training and education.

There are also skills that overlap: every welder that SpaceX or Lockheed employs is one less welder able to help install a water treatment plant somewhere. Here again, the solution is to train more welders.