He’s right.
Imagine a president were elected who immediately upon being sworn in moved to Fiji and never answered his phone.
There’s nothing illegal about moving to Fiji, or not answering the phone, but you would have to impeach him because he’s not a good president.
He's wrong.
In your example, if the president moved to Fiji and didn't answer the phone, then he would be in violation of his constitutional oath.
The Constitution is a source of law. Because the Constitution is a source of law, everything that is in violation of it is illegal.
I think we need to stop this Democrat anti constitutional talking point. Impeachment is a legal process.