The Rambam's first Principle of Faith is that Hashem is the Cause of all other existence and His existence is independent of all other existence. To me it seems the best source for this would have either been בראשית ברא אל-הים (Bereishis 1:1) or אין עוד מלבדו (Devarim 4:35)(which the Rambam cites at one point in the corresponding section of Yesodei HaTorah, 1:2). However, as his source-text for the Principle, he quotes אנכי ה' אלה-ך. Why does the Rambam quote this source?
(I have seen R' Shilat's suggestions to answer this question, and none of them are satisfying to me. The Rambam's Ikkarim are not mitzvos, and so quoting the source of the mitzvah of knowing Hashem exists does not seem relevant. I had issues with the other answers as well. I am looking for another suggestion to answer.)
I understand the Ikkarim like R' Weinberg did, as explained here, and would like an answer which fits with that model. However, an answer which assumes a different model is also acceptable.