Ah, but according to Trump he didn't know who the person was he was going to meet! And he never told Kushner or Manafort what the meeting was about!
So right after Trump clinched the nomination, when things were incredibly busy as they tried to get a national campaign off and running, and line up a running mate, Trump Jr invites Manafort and Kushner to a meeting and they don't ask what it's about or who it's with because, well, they were bored and had time on their hands anyway...
I believe it!