I am not making any excuses for Melania Trump, and yes, I’m sure she was lying when she said she wrote her speech pretty much by herself.
But let me reflect briefly on what an odd construct a political speech really is. You hire a speechwriter and read his or her words. If you’re a neophyte, like Trump, you probably just read what’s put in front of you. And if your speechwriter plagiarized, you’re a plagiarist. But if your speechwriter didn’t plagiarize, you’re not a plagiarist, even though you are passing off his words as your own—the very definition of plagiarism.