There really is no reason for him to know specific names right now. And it was a gotcha question. This is nothing more than fodder for the elite GOP. When Odumbo was running, did he get asked question like that? Of course not.
Remember in 1999 when GW was asked questions about generals? That was a gotcha question. Same with Palin and what newspapers and magazine she read and the Bush doctrine.
Believe me, this will not affect Trump supporters at all.
And everyone can Bing everything.