Of course J-10 fully loaded has no impact on flight performance?
It does of course but F-16 gives user the option to overload even more. People are assuming that the maximum load of F-16 (around 2T more than J-10) would perform similarly to J-10 at its allowed max load.
The difference here is the allowed max load of the 2 is different. I'm sure the J-10 can add another 2 tonnes but the performance would be more sluggish.
You are missing the important factor here. The difference is not that F-16 can carry more to the same performance let's call overall Performance level P1 (benchmark of high load performance) compared to J-10. It is that the F-16 allows user to load it more however Px < P1.
While J-10 simply enforces a minimum of P1 and does not allow user to simply load it further.
WS-10B and F110 have similar dry and wet thrust figures. Both fighters have similar drag and lift.
So why is F-16 capable of carrying more? The reason is in how much each side allows their fighter to have as max. With Chinese the allowed max is lower as the thinking may be along the lines of why should I max it out to the same extent as F-16 is allowed. It degrades performance and range far too much. Reduces energy for weapons released, cannot dodge missiles incoming as well, cannot fly as far or fast or turn or climb anywhere near what is required. So Chinese simply did not bother triple and double racking all the pylons.
F-16 allows this but most missions will not allow it to carry 6 BVR missiles, 2 WVR missiles, three tanks and pods etc. It's just a matter of what you want to do with it.
Some people still stuck at max payload. I suppose this is why those companies market their aircraft in beast mode.
It's the difference between going to combat like this.
and going like this.
Where both men have similar carrying capacity but both choose different limits.
