I own an old D2000/DCS 520. It is ISO less meaning ISO 200 or ISO 1600 will be the same when shutter speed and aperture are constant.
Original raw files end with .TIF but RawTherapee and darktable need .DCR to be able to edit them.
RawTherapee reads the files correctly.
If I don’t remember it wrong darktable worked like RawTherapee last time I tried, 2018.
Download the raw files, change TIF to DCR and compare in RawTherapee and darktable. You can also try
dcraw -E -4 -j -t 0 -s all 23HK3658.DCR to get a pgm file with all details in the highlights.