This is NOT a completely true statement.
IF there are no stops between origination and destination, you're fine.
However, if you have something in your checked luggage that is illegal in a country that is a layover/transfer/refueling stop, that country's customs police WILL confiscate it AND MAY throw you in the pokey as well. Case in point - Hong Kong - Hong Kong customs officials check ALL checked luggage passing through their airport, whether staying or just passing through.
Since Hong Kong's independence has gone the way of the dodo, I suspect the same would be true for a flight that landed in any other Chinese airport as well.
I have friends who bought knives that were perfectly legal in both the Philippines and the US that took off from Manila, landed in Hong Kong for whatever and they were subsequently arrested and spent a time in the local jail and paid big fines before release.