Two small fugures, the Archangel Raphael and Tobias, walk in the landscape below the Trinity. Their graceful style differs in technique from the main figures. It was normal practice for the artists like Botticelli to delegate portions of a large altarpiece to assistants. The angels surrounding the Trinity are certain
OK, the sentence makes sense in context. In this case "by members of his workshop" mean that the angels were painted by members of his workshop. And yes, "his" refers to Botticelli.