Disagree. I think a key thing in this saga is that the laptop was in his own seat pocket and apparently, small enough not to be obvious; also that there was other stuff of his at the seat. Again, I wouldn't leave my laptop unattended anywhere, but on a plane, at your own seat, and concealed to some extent I think its reasonable for the OP to expect that no-one else will go poking there and then hand over stuff when its obvious, from the other still at the seat, that the seat is occupied.
Once the item was in the crew member's hands, the crew member should have taken reasonable steps to ensure that it was in fact the previous seat occupants' and no-one on the current flight before handing them off the plane. Such reasonable steps I suggest would have been:
* See if there is any name in the items. I see there were credit cards, so there was a name to be checked against the current manifest (but maybe there are airline rules about poking about in people's kit);
* Approach the person in the seat where the items came from before handing them out of the aircraft. Remember they were in the seat next to the person who handed them in, so the crew member didn't know whether the seat was occupied or not.
* Make a PA announcement. "We've had some personal items handed in .. please check your seat pockets and surroundings to see if you are missing anything.."