I think he is using a definition closer to the one used in this paper: >Athletes experience minor fatigue and acute reductions in performance as a consequence

I think he is using a definition closer to the one used in this paper: >Athletes experience minor fatigue and acute reductions in performance as a consequence of the normal training process. When the balance between training stress and recovery is disproportionate, it is thought that overreaching and possibly overtraining may develop. However, the majority of research that has been conducted in this area has investigated overreached and not overtrained athletes. Overreaching occurs as a result of intensified training and is often considered a normal outcome for elite athletes due to the relatively short time needed for recovery (approximately 2 weeks) and the possibility of a supercompensatory effect. As the time needed to recover from the overtraining syndrome is considered to be much longer (months to years), it may not be appropriate to compare the two states. It is presently not possible to discern acute fatigue and decreased performance experienced from isolated training sessions, from the states of overreaching and overtraining. This is partially the result of a lack of diagnostic tools, variability of results of research studies, a lack of well controlled studies and individual responses to training. https://link.springer.com/article/10.2165/00007256-200434140-00003 https://link.springer.com/article/10.2165/00007256-200434140... So it is not the normal aches or reduced performance you get from training a lot in a short time, sleeping too little or anything like that.