This distance seems easily vulnerable to a stealth fighter's preemptive penetration?
It depends.
In conventional volume search mode, without prior indication of the target's location, the detection range can be this.
However, if the detection range is for a narrow-angle, full-power search, the detection range inevitably increases.
Let me give you an example:
Sukhoi claims the Su-35S can detect a 3m² target at 350-400km. That's a good range against an F-16 or Typhoon, but it means this latest Flanker can't detect an F-35 or F-22 until they're within 35-40km.
US fighters can launch the AIM-120D medium-range missile at approximately 80km, which is before the Su-35S can locate them.
This detection range is for a narrow-angle, full-power search; it depends on the F-35 or F-22 being previously detected by another system. In conventional volume search mode, without prior indication of the target's location, the detection range is half, i.e. 17-20km against F-35 and F-22.