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Lieutenant General
The question hasn't changed. Do you know whether the J-20 has launched weapons from its internal bay while travelling at supersonic speed? Deploying weapons externally is different from launching from internal bays. One can't assume is business as usual until is proven.
It's actually not always good logic to assume that something is negative unless you see proof that it is positive; the safest assumption is what is the norm and that is assumed unless there is evidence otherwise. For example, if you've never seen your coworker eat, the assumption should not be that he doesn't eat. There is no modern fighter (not in China or the world) that is incapable of launching weapons at supersonic speeds so to assume that of the J-20 actually requires more evidence than to assume that it can. If we were to follow your logic, we would have to assume that J-20 doesn't have a working ejection mechanism because we've never seen a pilot eject from it before.