Really? That's very interesting. What terms should I search for in order to find out for more about why negatives are so important?

Or is it just the fact that photography back then was done with negatives, and anything but the original photo is unsuitable for compression research?

a 512x512 pixel scan of a printed image really isn't ideal source material. Nowadays you definitely want something larger, and to get that in good quality you need the negatives.