Bolded part is what you've got wrong, the rest in true, Mamić did use his connections to elevate his status to national team in order to sell him and make money, but he was hell of a talent at youth level.
What they did wrong was pushing him to first team too early, he was never phsyically ready for senior football at the age of 16, should've let him develop properly, but both Mamić and Halilović parent got money hungry and basically ruined him in the long run.