On the one hand, it's sort of believable that he would not engage with literature of any kind. On the other, he kept saying it after being informed that the phrase came from Hitler.
When any reasonable person says something that might seem close to something Hitler says and is told about it, their reaction is to figure out how to re-express what they said more clearly so that it cannot be mistaken for agreeing with Hitler.
Trump's reaction apparently is to want to make sure you know he didn't steal the idea from Hitler.
