In the third person; it's not God saying "Then God spake all these words, saying." What follows is in the first person, because it's the quoted speech of God. "I am the Lord thy God..." and so on. in English we would use quotation marks, but Hebrew doesn't have them, so the only indication of quoted speech is the change of voicing. The voicing switches back to third person at 20:11, closing the quote. Now to settle this, one would have to know more about biblical Hebrew than both of us put together. But my point is, that what you offer as "proof" is really only conjecture and at that is built upon a theory of divine inspiration that not all Christians subscribe to. So, the verdict is, not proved.