'other gods before me' means a human or institutional representative, not a separate supernatural being. Neither polytheism nor monotheism are the subject of the Decalogue. The Decalogue is an expansion of the Shema, which explains that the One God is Alone and Singular, having no ordinal representative, like they had in Egypt.
The Israelites were not ready for the Shema, so God had to break it down for them into a set of rules.