Per Waite, the term "Hierophant" is like a general term for head of a church or religion, where "Pope" is specifically Roman Catholic. I think he did that a few times in his deck, used a more general term.
On the High priestess card she does appear to be sitting outside and also behind her you can make out water, which (i think) is supposed to represent the sea of the subconscous, the hidden knowledge that is locked deep within our minds. While the hierophant represents conscious knowledge and advice.