Not credible. On the other hand, 30 km is well within the WVR regime. Ground level visual horizon is in the low 20s.
Horizon calculator - radar and visual
If you want something a little bit more credible, try this anecdote...
Raptor debuts at Red Flag, dominates skies
Detection and Tracking are different modes of radar operations. Detection is the trespass of a signal over an arbitrary threshold. Tracking involve persistence over time
AFTER the target trespass of that threshold. So what this mean is that if the order of modes is: Detection-Tracking-Targeting, then even though the F-22 is 'detected' its signal may not stay above that threshold for long, making 'tracking' problematic and 'targeting' well nigh impossible.
Against 'stealth', the order must be: Tracking-Detection-Targeting.
Track-before-detect - Wikipedia, the free encyclopedia
TBD is difficult to implement on an aircraft, especially on a fighter class size aircraft. Basically, the threshold (clutter) is radically lowered to include just about everything. Then that 'everything' is recorded over time. Whatever that moves out of the ordinary is classified as a 'target'. The difficulty lies in data processing because so many things are being processed at anytime within the beamwidth. The solution is enlarge the data processing capability, which is not likely on a fighter, or narrow the beamwidth, which is still problematic because the beamwidth is already narrow at 3-5 deg for most and 2-3 for the more advanced AESA systems.
So even if we grant the Chinese speculation that the F-22 can be 'detected' at 30 km, it still does not make the F-22 a vulnerable target.