Sounds like the Chinese Air Force missed this pair. Better luck next time.
Too huge an object to be missed by even a dysfunctional radar.
If missed, then he Chinese surveillance system appears to be a failure.
Given the advance the Chinese have done in military technology, I don;t think that the Chinese radars are duff.
B-52 Stratofortress
Strategic Bomber
The Boeing B-52 Stratofortress (Model 464) is a eight-engine heavy strategic bomber aircraft with a maximum weapon-payload of approx. 70,000 lbs (31.750 kg) produced by the US-American manufacturer Boeing Company.
Crew 5
Propulsion 8 Turbofan Engines
Engine Model Pratt & Whitney TF33-P-3
Engine Power (each) 75,6 kN 17000 lbf
Speed 957 km/h 517 kts
595 mph
Service Ceiling 15.240 m 50.000 ft
Range 14.172 km 7.652 NM
8.806 mi.
Empty Weight 111.350 kg 245.485 lbs
max. Takeoff Weight 251.353 kg 554.139 lbs
Wing Span 56,39 m 185,0 ft
Wing Area 371,6 m² 4000 ft²
Length 49,05 m 160,9 ft
Height 12,40 m 40,7 ft
First Flight 15.04.1952
Production Status out of production
Production Range 1952-1962
Total Production 744
ICAO Code B52
Data for (Version) Boeing B-52H Stratofortress
Remarks
The Boeing B-52 is in service with the United States Air Force since 1955.