The differences between LSD and mescaline would really only be indistinguishable by users who had only used one psychedelic, very few times, or who had not ever used any psychedelics. I think I know the study to which you are referring, and that study used people who had had very little to no previous psychedelic experience. Once you get some experience in several compounds, you realize how vastly different LSD and mescaline are, and indeed, how different almost every psychedelic chemical is compared to almost every other. There are literally hundreds of them, and each is unique in its own way. But mescaline is extremely, fundamentally different from LSD in every possible way I can think of.
For one, mescaline does not tend towards ego loss. Rather, it tends toward ego amplification and clarification. This is probably the biggest difference but it makes the two very different chemicals. Also, mescaline is very, very calm compared to LSD. LSD is a tryptamine, and like most tryptamines, makes the thoughts race, and contains a significant "mind fuck" aspect that can result in a very difficult but also very mind-blowing journey. Mescaline is a phenethylamine, and although not all phenethylamines are like mescaline at all, mescaline itself is characterized by a very calm and meditating mind state, with actually not a whole lot of racing thought going on at all. The mescaline state of mind is vastly easier to navigate with calm and uniform emotion, whereas the LSD mindstate (and mushroom) consists of rapidly fluctuating emotions. If one were to graph the effects, it would be a series of sharp ups and downs, whereas the mescaline graph would be more of a smooth, gradual sine wave, never getting as high as LSD might, but also never getting as low.
Also, the physical effects are drastically different, although at low doses to inexperienced users, I guess I could see how they would be indistinguishable. LSD consists of an "electric" body buzz, whereas mescaline is much more earthy and warm and pleasurable (in general), as is typical of phenethylamines in comparison to tryptamines.
Finally, the visual effects of the two chemicals are very, very different. LSD seems to dramatically alter visual perception so as to view objects in seemingly more dimensions than our usual three. Colors abound, and rapid breathing and morphing of objects is common. Also, fractals are common to see at higher doses. The visuals from LSD are sharp and crisp, and well-defined, and detailed to an astonishing extent. On the other hand, mescaline's visuals are generally much more subtle. There is a slowly breathing of objects, and a faint crawling of small things like print, but the biggest difference is that mescaline visuals are almost cartoon-like, in that they make everything stand out in enhanced color and vividness, but they don't even come close to the sharp, detailed visual enhancement of LSD and some others. Mescaline's greatest visual characteristic is often described as showing you the world around through a lens of enhanced singificance, if that makes sense.
Anyway, I hope this was illuminating to you on the differences between LSD and mescaline. I think that study is extremely misleading in many ways, because as an experienced user, I find the effects of LSD, mushrooms, and mescaline to be vastly different, and I guarantee I could differentiate between them at a sufficient dose for a full trip with absolutely no problems. it also helps that I've used somewhere in the neighborhood of 15 research chemical psychedelics as well, which haver really helped me to understand the psychedelic state and the differences between various drugs extremely well.
If you ever have any other questions about psychedelic, or any chemical in particular, PM me or get ahold of me in some other way. or go to Erowid and find my reports by the Erowid Author name of "Lightwarden". or go to Bluelight and read my various posts as Xorkoth.
Also please keep in mind that my statements are generalizations, and every user will experience these drugs somewhat differently. But this should give a good general idea.