There is no ML histogram in Playback, so I don't know what you are taking about. Maybe you think the Canon histogram is from ML? Canon histograms are not based on raw data (again read the link)
I don't know about other cameras, but on my 60D, you can press unlock to get the ML overlay while in playback mode. I use it mainly for the peaking and the histogram. If I want to look at the Canon histogram, the image will be too small for me to see anything useful, so I prefer using the ML overlay
That's correct: in playback mode, ML only has access to the processed data, so it's not possible to produce a raw histogram, unlike in quick review mode, where the raw buffer is accessible.
Ah, that's what I couldn't figure out, thanks.
But this means that if I want to get as close as I can to a "raw histogram" in playback mode, I need to use picture styles to tweak the jpg preview.
EDIT: So I just tested Cinestyle and it is indeed the closest I get to a RAW histogram...In fact I can't tell the difference between the RAW histogram (from image review) and the jpg histogram (from playback)