Their claim does have support in so far that the early testament contains a lot of work written by polytheistic people that later in would become the monolatrists and even later monotheists that we know as Jews, further branching off into what today are Christians.

This does not mean that Christians in any sense are not purely monotheistic. Not only are they so, it’s one of the most critical parts of their beliefs, to the point where even believing that their one god has in any way shape or form some kind of tangible division is considered strict heresy from trinitarian churches which form the mainstream of Christianity and have done so for hundreds of years.

Edit: There is a great video by Alex O’Connor interviewing Esoterica on that topic in particular and they talk about the evidence that supports the viewpoints.