1) No one, not even the president, can be an expert on every few topic and people mistake when they say things, even presidents. It happens.
2) I don't expect Trump to be an expert on every topic. He misspoke. Oh well.
3) Presidents have expert advisers and are pretty much privy to any information they want, via these people (expert advisers). I do expect any president to check with their expert advisers on X topic before they talk.