remember the reason makoto swims? it’s to be with his friends, to be with haru. and that’s also the reason why he wanted to race him, at least once, he wanted haru to take him on seriously and when he does, this happens. I’m sure makoto knew that haru would win, but this scene isn’t about the race anymore, not for makoto. it’s about his relationship with haru and how, just like in the race, he is being left behind, bc as much as haru doesn’t want to admit it (or maybe doesn’t even realize this yet), makoto knows that haru will follow rin into the pro circuits and he will be left out.
thus, this race is makoto’s final test, he tries to find the strength to follow them, but seeing haru’s back, reaching his hand out and still not being able to catch him is the marker of his failure. and makoto realizes that it’s all over, he can’t follow haru anymore, he has to let go