I work in a Psychology department that has some members who specialize in the study of various kinds of bias, and I can assure you that unconscious bias is very real, and measurable in ways that I honestly can't find any reasonable way to claim measure anything else.

Look up the Implicit Association Test for a good example of this. I've taken it, and while it wasn't exactly a shocker to me to discover that I have an unconscious bias against black people (I grew up in an area that had maybe 5 of them, and despite my very liberal upbringing, our society and culture still label black people as "other" and "dangerous"), it was eye-opening to be able to see the effect so clearly.