He said new BotRK outclasses the BT, not IE. IE vs BT is a whole different debate.
And to answer your question: BT gives you a faster power spike and it's really good on AD-s with good AD scaling skills, like Graves. Ofc when Ashes or Caitlyns (God forbid, Tristanas) rush BT I want to kill myself, but I guess it's all about preference (maybe they want to do all the magic in early mid-game) and/or team comps (maybe they feel that the extra life-steal is crucial and willing to sacrifice damage for it).