Inroading is not a new concept. It was first coined by Arthur Jones in the 70s. It has been mentioned on and off by some people over the years, never really becoming a popular idea.
Muscle masse is stimulated when the muscle’s cell signaling pathways for growth are activated… in other words there is a series of chemical messengers that send the message to initiate the muscle-building process. Some of these pathways can be stimulated via time under load. So indirectly, fatigue can activate some chemical response.
But it is VERY reductive to say that fatigue is the stimulus for growth.
While sometimes you might get the idea that I am not someone who recommend fatiguing the muscles, that is not true. Sure I once said “chase performance, not fatigue”, but that is because if you train at a high level, you WILL induce a level of fatigue in the muscles and that most people are lacking in performance/intense contractions rather than fatigue accumulation.