He knows it fires up the people he most wants to appeal to. He's not crazy. Or if he is, he is in the same way someone like Nigel Farage is in the UK (one of his best buddies, as it goes). When you're going for the right wing, hate everything different, our land for people born here crowd, then... stuff like this is their bread and butter. They lap it up.
Doesn't matter how accurate it is. How much common sense it makes. It only matters how much it gets the people you want to agree with you... to agree with you. It's social media without the media. Probably anyone with a modicum of common sense looks at this and shakes their head. His fanbase though will be ecstatic. That's how populism works. Tell the people you think matter exactly what you think they want to hear. No matter how insane it sounds. It won him two terms.
You have to remember this is the guy who would have you believe that immigrants had an appetite for household pets and that people with covid should inject themselves with bleach. An ocean of salt just isn't enough to take with good ol' Don. Thankfully he won't be in office too much longer.