I disagreed because monotheism doesn't allow for worship of other gods.
Ah, I see the problem, you don't do what monotheism means. Monotheism means the belief in only one god. Henotheism means worship of one or a primary god while believing others exist. The Israelites and Hebrew semitic peoples were clearly henotheistic even if they only worshiped a single god (or were supposed to, anyway).
I don't know what you mean when you say monotheism was a much later addition. It appears to me that israelite belief in and worship of only one God was on the agenda as far as God and moses were concerned
If they didn't believe in other gods, there would be no proclamation against worshiping them. If no one had ever stolen anything, there would be no laws against it. If no one had ever used credit, there would be no credit laws or counseling.
Similarly, there were prohibitions against worshiping other gods and angels precisely because the Hebrew semitic peoples and Israelites believed in them. Baal was a great example.
(I'm not disputing the fact that the israelites did fall into false worship but, monotheisim had already been introduced and the first commandment didn't allow for worship of other gods as you suggest)
Well, false or not is a theological argument, one we aren't having. Either way, the first commandment NEVER EVER says not to worship other gods, just worship Yahweh before all others (or no worship together, depending on your view). Never, not once says "you can't believe in other gods".