[Q.G.]

Can you post a link to one of those Autochromes on the web?

Histograms do not have to be flat, no.
They have to be a good representation of the distribution and frequency of tones in the scene that was captured in the image the histograms are statistics of.
They also show whether good use is made of the full range the capturing medium is capable of. If all tones are bunched up at one side (i.e. the range is not used fully), you can perform a histogram equalization, or redistribute tones over the entire range in another way, but that rarely leads to better pictures.

So if the Autochrome histograms are flat, either something strange is going on with the Autochromes themselves, or with the scan that was made of these, or with the software that produced the histogram.