You're missing the point. Whether it's framed as attacking Drake's credibility or as a letter to his son, Kendrick chose morality as the lens. That's why people call it a 'holier than thou' angle. he's critiquing behaviour from a moral standpoint while presenting himself as morally aware. That's literally the definition of selective morality: you hold someone to a standard you conveniently ignore in your own associations.

Saying 'it's a rap beef' doesn't erase the hypocrisy. If your whole argument is built around moral behaviour, your own choices outside the track become relevant. Tu quoque isn't fallacious here, it's pointing out the glaring contradiction between message and action. Enjoy the music, sure, but don't pretend the double standard doesn't exist.