It's backed up in the report - it states that the Captain was convinced during the investigation that the problem was technical. Having known him well for many years, I consider this very likely.
As for inadvertent AP disconnects in FBW Airbus types, they are relatively common. But in 10 years and 7000 hours, I have never seen one caused by brute force. The effort required is huge - but easily within the scope of the seat to administer. The usual inadvertent disconnect is caused by an inadvertent touch of the disconnect button which, because if it's purpose, is necessarily available and therefore quite vulnerable. It can easily be done by an item falling off the tray table in turbulence. In my airline, an inadvertent disconnect is not a mandatory reporting event.