I think it’s all anecdotal, observed over hundreds of athletes. The idea originated with Joe Friel from Training Peaks.
I wasn’t a fan of it at first, but having used it with many athletes, it’s always been within ~5 beats of aerobic threshold heart rate when compared to a lab test. So pretty damn accurate.
Like most things in training, it’s probably one of those things that coaches discover and then the physiologists figure out why it works a couple of decades later.