Structure vs system
A little further on in the report (1.18.1, page 82) a definition of 'extremely improbable' is given as less than 1 in 1,000,000,000 per flight hour. This is the requirement for an aircraft 'system'. The experienced rate of fan cowl opening (Report 1.18.2, page 84) is 1 in 1.28 million flight cycles pre EASA AD F-2001-381 (hold open device) and 1 in 2.42 million after that AD. I don't know the average hours per A319/320/321 cycle but say it is about 2 hours, then the occurrence of fan cowls opening is about 200 times over the acceptable limit of a 'system' and it would be more of an issue.
In addition, if they were a system there would be a cockpit warning for unlatched cowls. However EASA reasoned that if you have a warning system you get false alarms