It's the appearance. As a politician, the most visible politician in the country, it does not look good for him to duck the interview. He keeps saying he has nothing to hide. He keeps saying he is not under investigation. Technically, you are absolutely correct that he doesn't have to do the interview. And as his lawyer, i would advise him to not do the interview. But unlike a criminal defendant protected by the presumption of innocence and the right to not self-incriminate, Trump is POTUS. Dodging the interview is definitely within his rights, but from a practical, political perspective, may not be the best course of action.
As to the above the law query, it was just that. If he is subpoenaed to testify before the grand jury, must he do so, or is the POTUS above the law? I don't do criminal law, but i don't think that issue has been judicially resolved in black-and-white, even though it has been discussed before in relation to other Presidents.