Matsuyama is his own swing guru
It is uncommon to hear of a professional golfer doing well on tour without having a swing coach. Matsuyama is from that breed and is all on his own.
While the majority might argue that it is always beneficial to have another set of eyes to keep the swing dynamics in check, Matsuyama takes full responsibility and accountability for his game and performances.
He doesn't give himself the luxury or opportunity of blaming a swing coach for teaching him in a manner not suited to him or not having him prepared suitably entering into a big event.
This is obviously working for Hideki and this unique situation means that he knows his swing and approach inside out. If things start going south, which happens to the best of the best, he can take a step back and correct what might be going wrong on the golf course.
He doesn't have to rely on going back to a swing coach post his round to fix the issue which means that he can potentially salvage his round and sleep comfortably at night without worrying about his swing for his round the following day.