They’re referring to the third stanza of the original lyrics.

And where is that band who so vauntingly swore ⁠That the havoc of war and the battle’s confusion, A home and a country should leave us no more? ⁠Their blood has washed out their foul footsteps pollution. No refuge could save the hireling and slave, From the terror of flight, or the gloom of the grave, And the star-spangled banner in triumph doth wave, O’er the Land of the Free and the Home of the Brave.

After a quick skim of the wikipedia article, the meaning seems to be a mixed bag, but most agree he was referring to the British, who fought with ex-slaves and had a history of pressganging at the time. Not saying Francis Scott Key was perfect or anything, but it seems more anti british than pro slavery.

Also worth noting all of this is not in the modern version most people hear. Almost all modern versions stop after the first stanza.