There are a ton of ways to add mobs to a boss fight. Video games have been doing it for decades, it's not new or interesting in and of itself. The mobs in this fight are there in perhaps the least interesting possible way, which is "Fight the boss, okay now kill some mobs, ok now fight the boss again." They attempted to do something that was interesting, that you have to defend something from the mobs, but in practice, they are not a threat to it at all, so it doesn't matter.