The phb is refering to a philosophical debate that asked if a tree, falling in a forest still makes noise if there is no one to hear. A modern day equivalent would be asking if a man, talking alone to himself in a forest with no woman to hear him, is still wrong.
Ratbert's philosophical non sequitur is a roundabout way of calling the phb a pinhead as kombi pointed out.
It an historic reference to when the Ottoman Turks overthrew Byzantine Empire in 1453. While Constantinople fell the Christian theologians were closed up inside the church debating how many angels could fit on the head of a pin.
I'm amazed that Ratbert even knew that. Or maybe with him it was just a wild stab in the dark and he actually hit upon something.