Look up the word "unsubstantiated". It doesn't mean false or untrue, it means unproven. Lately it seems that most of President Trump's claims are being substantiated.
They are not being substantiated. They are still being proved as false.
In the last Fox poll, 16% approve of Trump's tweets. 30% say he needs to be more careful about what he tweets. 50% disapprove of his tweets. When you consider that 30% strongly approve of Trump then nearly half of his strong supporters say he needs to more careful at best.