This illustrates how confusion exists about these terms. I don't really have a good answer to your question other than to say I think the term "plow layer" is a sort of informal term that really is not intended to impart a judgement about pedogenesis. There really seems no doubt (at least to me) that the main properties of an Ap horizon (especially those that are darkened by om) are mostly due to pedogenesis and it is a "horizon" in the technical sense, even though we often refer to it by the popular (non technical) term "plow layer". This is unfortunate and I'm sure confusing to those trying to learn about soil mophology.