Well, I may admit that there might be another possibility, too:
The engine was testbenched, overrewd and it blasted away. Our records don´t confirm this version (since we have proof that V-1 and V-2 were lost during bombing raids) but it is possible, yes.
Technically, the BMW-018 was well able to exceed 7.000 lbs of static thrust (diameter: 1250 mm, compression rate 7 to one, 12 stage axial compressor, three stage turbine combined with a very high airflow could well result in 3.500 Kp at 6.000 rpm), the Spitfire, however, was unable to exceed 1000 mp/h, we don´t need a proof here.The Nene was testbenched in october 44 and our records confirm that it eventually exceeded 5.000 lbs of thrust in early 45 on the testbench.