Yes it is true that there is little you can do to extend the dynamic range of the slide film.I was having this same confusion about ME Super's explanation. I thought I understood slide film decently, but now I'm not so sure...
With negatives, you expose for the shadows because nothing you do can recover information that wasn't recorded---you have to give enough time for the shadows to have some activated grains in them, and then you work with the development process to control the highlights. As is well known, slide film works the other way: If you overexpose and remove all the density, your highlights blow, so you have to expose for the highlights. But here's where I get confused.
Say I've exposed for a reasonable highlight (not a specular) in a scene with a long scale. Well, the shadow areas on the film are still receiving very few photons, so there are very few activated grains in the latent image. Doesn't that mean that I end up with the same blank shadows due to underexposure that I'd get in a negative? Changes in development can raise or lower the curve, but they can't create shadow information that wasn't there.
Does this mean that in practice very little can be done to extend the dynamic range of slide film? To extend the shadows you'd have to overexpose, but that blows the highlights; to extend the highlights you'd underexpose, but that gives you no information in the shadows; and there doesn't seem to be an analogue to compensating development that would help keep the highlights from blowing out. Have I got it right?