According to Patrick Pringle, who based his account on the original trial proceedings of Robert's crew:
"So about the middle of 1721 he went back to the Guinea coast. Here he ravaged the shipping of the Royal African Company, and by the end of the year his total number of prizes was over four hundred. In August he took the Royal African Company frigate Onslow, which he refitted and renamed the Royal Fortune, giving the previous ship of that name to the captain of the Onslow. He sailed with two consorts, Great Ranger and Little Ranger, and continued to cruise off the Guinea coast."
He goes on to state that in Jan. 1722 H.M.S. Swallow, commanded by Cpt Ogle, sighted Roberts at anchor off Cape Lopez. So Johnson was right, by and large.