They've got to be simplifying this a lot for the sake of making a press report understandable. They don't even know there's only one bit flipped. If there is one, they will have to do some analysis to figure out that it would really cause the problem.

Against what, Voyager 1? :)

They've got a live memory image, tracking down a possibly bad bit is probably not trivial. Given the era the code was written there could be self-modifying code in there too.

I'm going to presume that they know what they're doing rather than pretending I know anything at all about operating a 30 year old interplanetary spacecraft.

What I would guess is that it's a bit tricky to "just diff" an entire live memory dump (voyager doesn't have a hard drive, just ram and a no longer used tape drive), they basically need to debug the whole thing and figure out exactly what code lives where in memory, down to every last bit. I imagine this might be a non-trivial task.