"Gym rats" means anyone who's ever played a sport. Or done a job with manual labor. Or did any of many other things. BMI is "unscientific as fuck". It was literally never intended to be used anything like how it's used. It was solely intended to give a broad strokes single number for size relative to height (in a very limited initial population). There is no actual basis for its use anywhere.

The entire DSM was designed for the sole purpose of being used by a doctor. It was never intended to be used to self diagnose, or in literally any context outside of being used by a professional. The doctor's part of the equation is the whole point and the only thing that makes the DSM useful in any way. It is not standalone. It is a tool to enable doctors to have a consistent framework and process to do their job.