I would argue it really isn't clear-cut. Judaism has "sons of god" or angels who were each given dominion over one of the ancient human nations. The Hebrews were reserved for God. I suppose many modern believers see that verse as poetic rather than literal, but the many stories about angels in the Bible were clearly meant to be as historical and literal as any other stories. What's the practical difference between the angel Gabriel and the god Hermes? They both functions as heralds and messengers for the supreme god of their pantheon.
Christianity further blurs the meaning of monotheism with the divinity of Jesus and the belief that all Christians should become divine (theosis/deification). Jesus prays asking the Father for all his disciples to be one just as Jesus and the Father are one.
Furthermore, almost all polytheistic gods include a supreme god and a creator god (possibly dead or retired). Hinduism, for example, might be considered monotheistic with numerous perspectives on that one god provided by the other gods.
Judaism was able to evolve from its original polytheistic form to its current monotheistic form without ever admitting that a change had happened. That indicates a blurry distinction rather than a clear-cut distinction.
For that matter, where is the definition that distinguishes between divine beings and non-divine beings? If that definition is blurry then the whole concept of monotheism must also be blurry.
Abraham would not agree.
After escaping being put to death by Nimrod,
Abram was hidden in a cave, he stayed in the cave until he was ten years old.
During this time he came to believe in the existence of G-d through reasoning.
By watching the sun, moon and stars coming and going
each in their own time.
He reasoned there must be a power above and beyond
all the visible forces of nature,
a power that had created them and controlled them at all times.
Young Abram perceived the unlimited and timeless existence of G-d.
Then G-d made Himself known to Abram and taught him
the right way of living.
Later Abram went to the house of Noah and Shem,
he stayed there many years,
he studied and learned to serve G-d.
The Talmud tells that Abraham fulfilled the entire Torah
before it was given.
He, Abram had been taught the knowledge of the true G-d and he despised the idol worship of the people around him.