You know, something that I have always admired about Matsumoto is how hard he works and pushes himself to reach perfection. But once he understands that someone else's views have merit, even if it's in a different way, he never hesitates to take on what they say. So, in the story, once he understood that Aiba wasn't trying to suggest he was a bad designer, but that being child-focused required a different approach, it was easy for him to follow Aiba's lead. But Aiba isn't the best with words (especially without the context that a face-to-face meeting would have provided) so it took a long time for Jun to get it. He probably has a lot of respect for Aiba too, because of how seriously he takes his job.