The Pathfinder Chronicles Gazetteer, on page 13, establishes that they are paladins with no patron deity (such paladins being more common than paladins of Abadar, Irori, or Shelyn). And, of course, in straight D&D 3.5, paladins similarly need no patron god.
On the other hand, Divine Bond is specifically described as "divine bond with her god" on p.20 of Alpha 2, and Holy Champion (p.22) says that the paladin is "a conduit for the power of her god."
I'd suggest the name of the bond be changed to "Celestial Bond", it be described as a "bond with celestial powers", and the other be described as "a conduit for celestial power." This way, it works for both paladins devoted to gods and, in the words of the Gazetteer, "those who hold no single deity above others along the path of the righteous."
That makes sense, really.
I like it.