It was an exit. I seem to remember it opened in about 1990. That was the time when it took forever to vacate the Popside as you had to filter through the Cheadle End terrace and there was lots of pinch points.
When it first opened loads tried it out, but I seem to remember you could only go towards Moscow Road on the other side of the reservoirs and it was a really long detour for most. Thereafter, everyone stuck to the usual exits, and when that area became the away end, it was only used as an emergency exit.