Lisbon was closer...movement takes sooo long in civ vi that distance often determines best target. In my first sp game, I had the force to win the game quite early, 50 turns spent moving it into range. No losses on lisbon means his army probably gained from the fight because exp.
Played: FFH PBEM XXVI (Rhoanna) FFH PBEM XXV (Shekinah) FFH PBEM XXX (Flauros) Pitboss 11 (Kublai Rome)
Playing:Pitboss 18 (Ghengis Portugal) PBEM 60 - AI start (Napoleon Inca)
Playing:Pitboss 18 (Ghengis Portugal) PBEM 60 - AI start (Napoleon Inca)

That being said, with the defensive ability of cities and the slow movement makes it pretty hard to attack a human player early on.