I think the punchline is just that he doesn't get to show the results of his training more (as he'll be stuck in the back), but yeah being a goalkeeper doesn't mean that they shouldn't learn the fundamentals.Wait a minute. If he was weak on soccer fundamentals, the training still helped immensely at goalie especially for clearing the ball back to midfield (we see that he was weak at kicking before).
Plus Goalie is a high visibility position. If they win, he did a good job there. Maybe goalie isn't as sexy as being a forward who scores but it's still presumably easy to get credit.
The punchline doesn't land for me. Is there a Japanese cultural thing going on here where goalies aren't seen as popular?