So in the long history of basketball, successful coaches for years have used timeouts to stop an opponent's run of points along with re-setting their own team's offense and possibly making a defensive adjustment. The timeout also serves to hopefully cool off the other team's shooting and offensive flow. Another positive effect is the pause which allows a raucous home crowd a chance to quiet. All these probable results of calling a timeout are positives for the coach's team.
But along comes Joe Mazulla, youngest coach in the league and with no head coaching experience aside from a small college stint a few years back and he has a magic formula for improving his team and supposedly winning games. He refuses to call timeouts in the middle of large scoring runs of 10+, 15+, even approaching 20 points by his opponent. This guy, with absolute minimum experience, is going to fly in the face of convention and, according to him, develop his team's mental toughness by leaving them out in a blizzard.
Yes, the record is great vs weak to mediocre teams. I see lots of slippage into bad habits in the middle of these runs the Celtics have endured. Why are you trying to be some genius Coach ? The offense has improved when they don't self destruct in a barrage of missed 3's that you do not put a stop to. Stick to the basics and focus on what you seem to be good at.