Trump's lie about dead troops/presidential calls is not only insidious, it perfectly encapsulates who he is as a person. Not only is he deplorable, he isn't intellectually curious and will believe anything he's told. There's an entire network of news dedicated to influencing him with bullshyt. No doubt some aide with ties to the alt right told him this bullshyt, and he repeated it hook line sinker.
Then notice how, when faced with the truth, Trump refused to accept responsibility, said he was "told" this and didn't look it up. THEN he insinuated the generals told him it, which we know is not true.