I did that once at JFK. I was afraid that the rigid suitcase would be mishandled so I anticipated. It did end up mishandled: I don’t know what they dropped on it but it had a large dent when I got it back. I was also checked at customs, where they cut the suitcase open with a knife, at which point I realised that the closing mechanisms were smashed and I could not close it again (they gave me some duct tape to keep it somewhat closed, but the following train journey was a nightmare). That was fun. But I did not regret the industrial strength cling wrap.
My first thought is to type "WTF!", however having experienced the bully-boy antics of the US police (and how the US population appears to take this for granted) this behaviour by your Customs does not surprise me.
I believe that the comment is referring to cutting off the cling-wrap to open the case, something that would likely be done by customs in any country.
The simplest solution to a broken backpack buckle is another buckle. If you’re going to carry something, carry those. They’re light and cheap and should be part of the repair kit for backcountry travel.
You can also simply thread the strap through the webbing loop and tie it off. Hard to get the length exactly right but works in a pinch.
Problem is, cling wrap won't stop a determined thief, it'll just make the theft evident -- unless they just grab your whole bag instead.
The point is just to make your bag more effort than the less-protected one next to it.
And that is the day that I learned the Miami airport has a service that will basically use industrial strength cling wrap to seal your luggage.