I don't see Arsenal as part of the contenders in this season title race. Arsenal should first of all, win EPL before they can start chasing the Champions League title. Barcelona isn't showing too much of quality in them this season. Maybe, I am using the few matches that they have played to judge them but they need to improve their performance if they want to be part of the Champions League contender this season.
Winning the Premier league trophy should come first before considering to win the Champions League and that is definitely no easy fit with the likes of Liverpool, Chelsea, Manchester City, Real Madrid, Barcelona and the rest. Arsenal can be prospective contenders but winning the trophy requires another level of experience entirely.
I definitely agree with you Arsenal main focus should be how they can win the domestic league first before considering challenging for the champions League because it's been a while that arsenal tasted premier League victory this will even boost their confidence going into the champions League knowing well that their moral level will be high.