Is that type of pylon exclusive to PL-15 and unable to carry PL-12?
I'm not quite sure if that's a smoking gun evidence that allows us to say "J-11BG is compatible with PL-15" definitively.
Did a little digging and so far no decisive evidence that this pylon is exclusive to PL-15.
However, there are indirect evidence pointing toward this probably being true: apart from this image of J-11BG only J-16 has been spotted with this particular pylon design (non of the flankers we know for sure don’t use PL-15 has been spotted with it), and whenever this pylon is seen it’s either empty or carrying a PL-15, the J-16 always uses something else for PL-12 as far as I know