That is an unsubstantiated comment , unverified, by someone reading the article. There is no proof given it is true. Even if there were fallen angels- demons in the painting, this does not mean what Shirly claims it to mean.
Demons are fallen angels who are known to tempt humans. There are plenty of them in all forms of art. An artist would be most likely painting them in to represent good and evil in the world and its fallen nature.
The artist might be representing the good angel, the guardian angel, protecting children from the evils of the water and the bridge and death.