It's easy to claim a given number, but are they backed up by tests?
They should be doing some test as it's the radar that the fleet will trust their life to.
What they publish tho is up to them. 400 km or in excess of that range figure is well within the ballpark of radar of that size in 052D. It will of course depend on target RCS etc. Typically however S-band radar will have range advertised against 2 sqm RCS with specific PD (Probability of Detection) and Pfa (Probability of False Alarm) which typically 50 % and PfA correspond to a swerling 1 target of 10^-6.
IF you really interested in digging, you have to at least have rough idea how big the antenna is, and find some reference of power of TRM's. Back in 1990's China already have as far as i remember 40 Watt AESA TRM in S-band (3 GHz). Then estimate the TRM numbers from the antenna size. If we can assume it's as big as SPY-1D you can expect 4350 TRM's Maybe bit less as the radar also contain extra arrays for C-band datalink for HHQ-9, can maybe take 3000, this still more than DDG-X Zummwalt's Volume Search Radar which have 2268 TRM's. You can then try using radar range equation to get the range estimate.