That's not showing any underexposed shadows, at the same time as the overexposed highlights.
We haven't designed it to do this yet, which is they are still two separate options.
My screenshot was intended to show that Photo Mechanic could show you the blown highlights on a RAW image after a preview has been delivered by the RAW render (implying that showing shadows was also possible if I changed that setting). If I have Photo Mechanic set to 'always render the RAW', then I see the histogram after the render has finished, and the histogram is for the image that the render produced.
If you are not seeing a histogram for your images, regardless of your raw rendering settings, please provide an example.