Well the belief that every God and Goddess is just the Wiccan God and Goddess isn't universal. I've studied Wicca for over 10 years and I've never believed that [though it is an interesting theory] same with the Horned God being a Sun God, while I believe that, this is a new belief. When Gerald Gardner first spoke of the Horned God, he was not associated with the sun. So the belief of the Lord being the sun isn't universal either.
Some people do claim certain deities are like the Goddess and God and therefore can be called on in much the same way [such as Isis, Hecate, Pan, Ra, Freya, Oden and basically any forest or maternal deities] The Goddess and God are rather complex, being a part of everyone and everything, so some may choose to see them as pre-established deities, others may not. Also, they can have a 'dark side' to them. While most Wiccans would not call on their darker elements for revenge or something, they can be vengeful, so you could associate the Goddess with a Goddess of war if that's how you viewed her.
Of course all my points are eclectic in nature, a Dianic Wiccan would completely shun the God, so their view would be different. A BTW [British Traditional Wiccan] might have an entirely different view. The belief all deities are the same deity is just one of many beliefs. Personally the Triple Goddess and the Horned God are separate, though share similarities with other deities. I work with the Horned God, but I've never felt a strong connection to the Triple Goddess, so I searched elsewhere. I still believe she's real, just not the Goddess I'm meant to work with.