I think you should read the stuff one more time: well, do it first! In the english part it is written "
This He-177A-5 (GP+RY/Wk-Nr 550256) was modified to become one of the few He 277/4-7 pattern aircraft in summer 1944."
Another point:
Group Dor was a FFI resistant group organisation INSIDE the SNCASE to build/repair/fly the machine they have at Toulouse, it wasn't the French Air Force. It became GB 1/31 "Aunis" in December 1944. So yes, this machine wasn't flown by the French Air Force.
And I wrote - considering the Royan pocket: "I'm not sure though".
I'm not here to write book criticism specially about He177, but considering Butler Bf109's captions I will not give one penny for his books.
Regards,
Franck.