I understand that Plato himself may have had nuanced views regarding divinity (the whole discussion of the One/the Good and the Many etc. which the Neoplatonists take up is arguably monotheistic.) This is not my question. My question is not about Plato's views himself, but the way he seems to mention "the god" (as if there is only one?) in casual settings as if this was understood by all Greeks of his time period.
Here are some examples: in the very beginning of the Ion, Socrates asks, "Do the Epidaurians hold a contest of reciters of poetry in honour of the god?"
Who is "the god" here? Socrates is talking to Ion here, who is supposed to be a polytheist, right?
Again, in the Apology (which is supposed to be more historically representative of the actual Socrates than the other dialogues) we find copious mention of "the god" while he is addressing a court of 500 Athenian jurors. Who is "the god" here?
I answered this question a while ago here. The use of "the god" doesn't imply a belief in just one god, but refers either to a god that can be understood from the context (which might also be "the goddess" when appropriate), or to a divine force which the author doesn't want to name. It was a convention of prose writing (as opposed to poetry, which was thought to be divinely inspired) to avoid claiming knowledge of the divine; even where something was clearly thought to be the work of some deity, it would be unseemly for someone who was not a priest or a seer to pretend to know which one.