Is this not usually the case?
No.
some of whom may have noticed the cowls unlatched (and flapping?) state
Did any other aircraft notice the cowls unlatched or is this pure supposition on your part?
The report goes on to say that one of the immediate causes was that the aircraft was signed off as serviceable - which is where the Captain's signature comes in. The aircraft is not allowed to fly unless the commander has signed to say that it is - regardless of engineering activity.
And if the Tech Log entry had said "IDG oil replenished" would the Captain personally open the cowls and check the oil levels for himself? The Commander cannot personally check every aspect of the aircrafts airworthiness.