It's his mistake because he didn't call her on it, and agreed with her premise by saying: "OK." Thus he validated what she said.
We’re seeing it differently. OK was calling her out. As a parent to a child. Which is why newsie changed the subject instead of defending ‘all of them’. Trump leads in some state polls, is behind in some state polls and is a statistical tie in many national polls. She made a stupid statement.
Pew 41/37
Rasmussen 41/39
Reuters 39/35
economist/yougov: 41/35
NBC: 43/37
Brietbart 42/37
LA Times 44/43 (two way race)
Rasmussen 43/40
Bloomberg 44/40