And Pope Francis is simply wrong to assert this.

Development of doctrine is not change in doctrine.  The pope cannot change doctrine, then call it "development."  When doctrine develops, it grows, but as it grows, it remains consistent with itself. 

Until Pope Francis, the Church has consistently taught that the death penalty can be morally justifiable in certain circumstances. Pope Francis, now, is seeking to reverse millennia of teaching and create a new teaching, namely: that the death penalty is an intrinsic moral evil. 

If Pope Francis has the authority to change church teaching on the death penalty, it is not necessary to appeal to development of doctrine. Just change the teaching.