I don't know what are those cans and jars of healthy food you are referring to. The only thing I can think of is canned fruit/peppers but they are definitely more expensive than the caloric equivalent of junk food.
Also as the article said, if you have uncertain income you will end up going on a starve/binge cycles which are also guaranteed to make you fat.
Another issue is that any affordable healthy food usually requires cooking and many poor people (esp in urban areas) do not have access to clean kitchens.
So it is a real issue and saying "they are poor because they are stupid" is pretty fucked up.