Well I don't think it necessarily means that Moses didn't write it. Moses' writings are very poetic and he uses poetic language. The fact that he chooses to make the distinction between an Almighty God and the Self-existent God might simply refer to the perceptions our patriarchs had of God. He might be inferring that there was hitherto a lesser understanding of God, yet now God would reveal more.
The same even applies today; the racial view of our ancestors was very primitive in that in their recordings God mentioned them in a nationalistic sense, not a racial sense. When it came to who was racially pure and who wasn't, they seemed to be mostly clueless. I mean, when they conquered nations -- Idumea in 150BC being a prime example -- they thought if they became circumcised they'd become Israelites. Christ said, "You must be born of water and spirit", which our earlier ancestors clearly didn't understand. Fast forward to this day and age and we have a rather mature, even complete understanding of what we ought to be doing, even though we are prophesied to still have a hard time telling the difference (wheat and tares).
If a holy, moral mindset is in a poetic sense to have the Holy Spirit, and given how the Holy Spirit is likened to God, it logically follows that we have a better understanding of God today than what our ancestors did. This is my point and what Moses might have been referring to. Does this mean we have another word for God? In sense, yes, in our Deity Jesus Christ. Our understanding of this Character of God has given us new understanding and a new name. We are even called differently i.e. "Christians".