From Wikipedia: "A role-playing game (RPG and sometimes roleplaying game[1][2]) is a game in which players assume the roles of characters in a fictional setting. Players take responsibility for acting out these roles within a narrative, either through literal acting or through a process of structured decision-making or character development.[3]Actions taken within many games succeed or fail according to a formal system of rules and guidelines.[4]"
First sentence: Player assumes the role of Link in a fictional setting. Check
Second sentence: There's structured decision making and character development. Check
Third sentence: The developers put in rules about what items solve puzzles and beat bosses. Link may fail if he does not use the right weapon on a boss for example. Check
Just because you can't make Link a mage or a healer doesn't mean it's not an RPG. An absence of detailed character customization doesn't disqualify it from being an RPG.
Also who cares lol. Sometimes I think people just think the Legend of Zelda is too good to fit into any genre.