Most animals reach satiety relatively quickly when it comes to food and don't overeat. With drugs satiety is hard to control or achieve. The article mentions

Most animals reach satiety relatively quickly when it comes to food and don't overeat. With drugs satiety is hard to control or achieve. The article mentions the zebra fish exhibited drug seeking behavior. The researchers almost certainly know about addiction and understand addicting behaviors. I think one should assume the researchers are aware that drug addicting behavior would be different than pure classical conditioning. If you or I, laymen on a topic, can think of a possible flaw after a moments thought then we should assume the experts have likewise thought of this possibility and have accounted for it in their study. The writer of the article may not have accounted for it in their explanation though.