So, what we call original negative is not the image directly captured by the camera for each shot.
It can be, or it can be a dupe with the camera negative in stored away. Some films even have more than one ON. Profondo Rosso/Deep Red may have had two entirely different ON’s - I don’t know I haven’t looked into it, the alternative is the director had the ON reconfigured for export. My understanding is it would have had an export ON seeing as the 4K restoration was made from the ‘original negative’ implying that the Italian Cut was intact.
Can a 4K scan of a very good print be as sharp as it is ?
Well clearly it can be. Prints do get scanned and released on Bluray when there’s no ON or IP available. Studies will use whatever they can, depending on the condition and their budgets, but they prefer using the ON or the camera negatives where they can - Robocop 4k was scanned from the camera negative for example. Obviously that could be much more expensive and time consuming than simply scanning the ON because you may have to first catalogue all the camera negatives and find all the pieces that were used in the final cut of the film before you can even start work on scanning it.