That's an interesting one, Supercarp, and one i hadn't seen up until now so thanks for posting it.
Look at the "beam of light" coming down from the clouds.
I'm of the opinion that what were called angels back then were actually ET's. The wings on the beings aren't an actual depiction...i think the beings are depicted as having wings to show that they had the power of flight.
Just like many civilizations and cultures have done and still do.
The Sumerians had bird-headed Gods who had wings too, the Ancient Egyptians had the Winged Sun disk. In fact Many ancient civilizations had a winged Sun disk and sometime even had one or more people riding in the disk.
It seems you may be right, Supercarp.
Maybe this chap had an encounter with some angels. De Gelders paintings do seem to depcit unknown beings of unknown origin, saucers and ET releated phenomenon, and i don't think this is by accident or coincidence.
He must have had good reason to paint these images.
Peace, Caper.