Hey there, over its lifespan the game has had several sweeping changes to things like character progression and unlocks. When it launched in 2017 it had an RPG-like character level system which gave unlock points towards subclass abilities. 20 was the original pre-DLC cap. This system was abolished when Shadowkeep launched in late 2019 - while some other achievements have been adjusted to work with different requirements, that particular one just gets awarded automatically as the level system it once worked with has long since gone. I see you have an old Hunter on your account from 2019. I suspect that was what triggered the Hunter trophy, not the Titan itself, just the act of loading into the game world with that old Hunter on your account.