If it is, then all my theories just flew right out the window.
I'm pretty sure that it is ported vacuum. I did some hands on testing last year, and uncapping that port at idle had no effect while uncapping most of the others affected the idle. Based on that, I figured that it was ported vacuum (it did draw once you tipped into the throttle).
If I am mistaken, please correct me before I continue to spread this sort if disinformation. Thanks Carl!