IMO you are reading too much into a plastic model, the yellow bits could just be casting connectors or flow channels used for casting. Like those little plastic bits in model plane sheets which keep all the parts connected to the sheet so none of them fall off.
I mean, the whole point is that this is an official AVIC commissioned model where the details deserve careful consideration.
If this was just a random model on eBay or Taobao made by nobody then this amount of scrutiny would be a waste of time and completely inappropriate.
There have been many models made of J-20 over the years, some with details that were obviously different to the real thing, some depicting the aircraft with a ventral bay carrying six missiles as well. But those models were not given any consideration or serious thought at all because they were not official.