Sometimes if the oolong is aged as well as roasted, this will contribute to not opening up too, but if it's a new tea it should unfurl some. Not as fully as green TGY or gao shan of course, but some. But yes, these are general guidelines, not hard and fast rules. The old Classic Roast
from Tea Gallery was an exception. It barely unfurled at all, but the quality of the material and the roast were both fantastic. Looking at the color and texture of the leaves, it's still quite different than that "oil" oolong though. I think the simple fact they called it "Oil Black" oolong (very unappetizing) should be a clue to stay away- to me that says "oops we burnt this batch, let's give it a name that implies we meant to do it this way."
I once had a 25+ year old master roasted dong ding, which was re-roasted every couple years for its life. After brewing, the dark leaves unfurled almost completely, and you could even tug and pull at them without ripping. Still one of the top ten teas I've had to this day, and an extremely rare level of roasting mastery.