If in this case they modified an existing backdoor to point somewhere else, evidence of the backdoor would also expose the original.
> The United States had gone to war declaring it must destroy an active weapons of mass destruction program. Instead, American troops gradually found and ultimately suffered from the remnants of long-abandoned programs, built in close collaboration with the West.
Nobody ever claimed Iraq had no WMDs, since that was in no small part what the first Iraq war was about. The claim was that Iraq was actively developing WMDs and that the US+allies had to intervene in the 2000s, a claim that has been debunked heavily.
I'd go so far as to call it an established fact that no WMD program existed that required international intervention in 2003. Most of the evidence otherwise has been debunked, and no additional evidence was found during the war itself.
The press coverage, of course, leaves you free to let your imagination run wild.