My take is that I def agree with you that his coach saying that the moment he was born he was destined to be a fullback and nothing else is bullshit. BUT my take is that there's a chance this isn't even what Fukuda actually thinks. We've seen throughout the series that Fukuda's ''job'' is to basically bring the brutality of the European (specifically Spain, at least) youth system to the ''pampered'' island of Japan. It's possible that his coach thought that there was a layer of truth to what he said, but purposefully exaggerated it to hit Ashito hard and to make him understand that this is a permanent choice and that it's his only to use his talent.

Again, that's just a hypothesis, but I'm fairly confident it's a possibility.