"Assault weapons" have no definition.
Assault weapon
In the United States, assault weapon is a controversial term used to define firearms with specified characteristics. The definition varies among regulating jurisdictions, but usually includes semi-automatic firearms with a detachable magazine, a pistol grip, and sometimes other features, such as a vertical forward grip, flash suppressor, or barrel shroud.Wikipedia
Huh. Live and learn.
That admitted, which RFK Jr. probably already knew, his response made good sense in the context where it was raised: (1) 2nd Amendment rules; stop trying to subvert it (2) IF Dems + Repubs unite to pass legislation to restrict 'assault weapons' he will sign it. Those are a lot of obstacles in the way of anything getting signed, which should give all concerned some counsel.