Yes, and I've now had a reply confirming it's like this.
The article was from a personal report of someone fined for having shopping on the back seat of a car in north Tenerife. The person speaks Spanish and understood the police officer, who said that the items were not properly secured.
There is nothing in the law specifically about stuff on the back seat, but given the requirement for dangerous loads to be secured, and in the current environment where fines are being issued for little more than breathing, it's as well to be cautious.