I think that what qualifies as a "main quest" differs depending on the game. If the game lists it as a main quest, it's a main quest so long as its necessary. But if there is a mission that is absolutely essential to the completion of the game and is not listed as a main quest specifically, its still a main quest. They simply didn't identify it. BotW has a different definition of "necessary", seeing as almost everything is optional. I would say what they listed as main quests are, in fact, main quests. That's just because they are essential if you're playing the game straight.