'Thunder' is what Pakistanis named it. Before that, while it was being designed and developed and built in Chengdu, they used to call it 'Xiaolong' - ie, "fierce dragon". But it wasn't fierce enough for the dragon's air force, so they sold it off to a lesser air force that had lesser ambitions and lower expectations. Those poor sods not only believe that it is the greatest thing since baked bread, but also that they designed it themselves.
As a consequence of that mistaken belief, they often come to troll on any thread related to India's efforts to build a fighter indigenously. Ending up belittiling something they can never accomplish.