A Successor can also transmit someone else's words. Notice the distinction:
• The Successor's own saying or action: a maqṭūʿ report.
• He transmits a Companion's own saying: the words belong to the Companion, not the Successor; this is mawqūf (موقوف).
• He transmits a saying of the Prophet ﷺ: the saying belongs to the Prophet ﷺ, not to the Successor himself.
Knowing that a narrator is a Successor is not enough. First establish whose words or action are reported.