The concept that Scripture is the ONLY source of authority in the Church is the error of “sola scriptura”. There is no prohibition on referring only to Scripture to answer specific doctrinal questions if the occasion allows it.
It's not necessary to always refer to every possible papal teaching, etc. when engaged in apologetics. Sometimes (or even often, for that matter) a Bible verse/s will suffice.
Catholics believe that Scripture and Sacred Tradition are two parts of the same Deposit of Faith.
http://www.catholic.com/library/Scripture_and_Tradition.asp