No they're not, they're very good at retaining possession but everytime they get caught on the break or are under pressure; their defense collapses; so far they have lost to both Liverpool and City who have the best attacks in the league.
They're like Barcalona, their defending is retaining the ball possession; the only difference is, Barcalona bore their opponents into conceding, Arsenal actually go out and attack lol.
If you base everything on your logic then nobody would be good at defending. Also if you look at many of the recent home games against the likes of Palace and Cardiff and away to West Ham they have scrapped those wins out, but kept clean sheets in the process, which says to me they have put more focused on being tight at the back and being petient when it comes to attacking. Are you seriously telling me than the two centre half's have not had great seasons? If so I don't know what Arsenal you have been watching.