I assume that is an array because the plane has an EW function and they removed the gun to support that. What would they remove the gun for? Its not to put composite, which may already have happened previously. You remove the gun to obviously put something there and the most likely thing to be put would be an EW support array.
In the history of EW specific aircraft like the Growler, things are installed inside the plane to support this function, which makes them a bit more than a fighter carrying EW pods.
To admit I don't remember this discussion and I for my part find it quite useless. To assume it is an array since the Chinese don't like Titanium or replace certain parts with a domestic solution so that by this conclusion this must be here too is IMO wrong.
IMO - esp. if you look at the countless close-up images available - it looks exactly the same as on any other Flanker, and therefore it is the same: Just a metal (Titanium) piece in that area since it is probably simply the best material in that area.
And that's it ...
Images only show the root of the wing to be grey, not the side under the cockpit, which the J-16s are. Not to mention using color and shine is an unreliable method since composite structures for aircraft are indeed silvery and grey, due to the use of carbon nanofiber.
As for the Chinese not liking Titanium, there is more to that. Flankers are flux welded Titanium, and to do that, the parts needed to be done inside a room filled with Argon gas. Without using Argon gas, any oxygen and nitrogen that goes into the weld would eventually cause the welds to crack. The whole process is tedious and slow, not to mention expensive. The use of a special argon gas facility puts the manufacturing into a bottleneck, not to mention you are going to be competing with other applications of titanium welding that includes various commercial uses. Another thing is that the Russians have a special procedure of their own, call it a secret sauce if you will, for flux welding titanium for the plane, one of the things they appeared to not have divulged to the Chinese as part of their trade secrets. They hoped that they would eternally provide components for the aircraft, and keeping the Chinese dependent for Russian provided kits of these essential sections (J-11A), and under their political influence for that. Attempts by the Chinese to replicate the process on their own failed to attain the same level of quality.
In contrast if you are already supplying aviation companies with composite sections, like for commercial aircraft, it wold be natural to reverse engineer the aircraft to make better use of your production competence and facilities, reduce your costs, bypass any trade secrets held from you and free your production bottlenecks. In the long run, the replacement of composite is superior to titanium in long term durability, not as equally subjected to hairline cracks, and provides the combat advantage of helping to reduce your radar cross section.