If the rock came from Shayol Ghul but it did not matter, it seems kind of odd Jordan would spend any of his precious and fleeting last moments jotting that down for Sanderson to somehow include (or not.) "I did not have time to tell Brandon how Moiraine is rescued, because I was busy letting him know the irrelevant-but-cool fact the Black Towers walls came from Shayol Ghul." It is equally hard to imagine anyone going to the trouble of obtaining that difficult-to-obtain material (how was it transported? Traveling near Shayol Ghul is known to be dicey, at best) if it were not useful for something. Maybe it is just something trivial like allowing the DO to directly observe the BT, or perhaps it does something more significant like allow him to extend the powers he has at Shayol Ghul to the BT. I cannot imagine it being completely irrelevant since it was specifically stated though.

Why Sanderson did NOT RAFO this is the bigger question to me. Unless he just thought, Well, there is no reason it could NOT be, so as of now it is, just to send them off on a wild goose chase so they do not puzzle out anything significant in the next six months. That would not be dishonest or deceptive, only misleading (which I consider legitimate when people fish for confirmations that are also spoilers) but it would also be kinda weak.