They are wrong.

21 gun salutes are for The President of the United States, former Presidents and Presidents-elect.

AND

Chiefs of state, heads of government and reigning monarchs.

Also, the number of guns and the interval DO matter:

https://en.wikipedia.org/wiki/21-gun_salute#United_States

A three-gun battery firing two of its guns with five-second intervals between rounds and one gun remaining at the ready in case of a misfire; such a battery would be used at an Armed Forces Full Honors Funeral, or for State Arrival Ceremony of a foreign dignitary at the Tomb of the Unknowns in Arlington National Cemetery.