Rabbi Dovid Rabiat is brought in the footnote (#11,12) of your Pathways article as a source for explaining the idea of avoiding the smearing or spreading of foods for simply aesthetic purposes, on Shabbos.
Rabbi Dovid Rabiat on page 921 and 925, points out that in order to ever violate the law of "Memarayach" (smoothing) on Shabbos, one would need to apply lateral action (a smearing spreading motion back and forth) to the substance in question. Simply "pressing" the substance vertically, (flattening it or changing its shape) is not "Memarayach" at all.
One of his sources for this is Shulchan Aruch 316 with Mishnah Berurah 51. He brings Gemara and other halachic sources which say or show the same thing.
Furthermore, the general thing that is to be "smoothed" needs to be mushed out of its current hard or pliable shape and change into a spreadable substance. As far as I can tell, this does not happen with rice (being made into sushi). Even if this does happen to the rice slightly as part of the process, since it is incidental and not the aim of the sushi maker, it is permitted.
However, spreading peanut butter, egg salad, icing, etc. is done with a smoothing action. The Magen Avraham (brought down in Shulchan Aruch 321; see Mishnah Berurah with Beur Halachah) says that it is ok to spread cooked apple filling over an area of pastry that is empty of the filling. (This is like spreading peanut butter on a bagel) This is because the apple mush is edible anyway, as is, without the smoothing; and you are presenting the food in its usual way to make a sandwich and eat it. (Furthermore, there is no possible "D'oraisa" of "Memarayach" with foodstuffs used as food. The only question is if there is a "D'Rabbanan" to be applied.)
There is an argument among the poskim if there is a D'rabbanan here if the spreading is purely for aesthetic purposes. It seems that those who forbid rely on the SeMaK. However, the Rema simply says: "Whoever avoids it, should receive a blessing." "Tavi Alav Brachah." Essentially, the Rema is unwilling to forbid, so the lenient opinions rely on the Rema.
Interestingly, the pathways article brings R' Rabiat in its footnote. He actually says that "it should be avoided", and there is merely "a firm basis for Rabbinic restriction to be applied." pg. 927-928. His footnote #91 on that spot simply refers the reader to the Rema! (the lenient source!Shulchan Aruch 321:19) R' Rabiat therefore does not forbid it. Your pathways article may be assuming the general stance of the more stringent poskim who apply a D'rabbanan here (at least) to aesthetics. They may be quoting R' Rabiat as a source in English that can better explain the reasoning behind the potential "issur".
In addition, shaping the sushi into rounds, would not be "Boneh", "building" (like making cheese which would be an act of "Boneh" because the particles are pressed into a new hard single mass etc.) since the smooth looking rounds are a simple circle shape (as opposed to an animal shape or a tower or sculpture of a person in chopped liver etc.) and lack having the particles pressed into a new single form. It is also not "Kotev", "writing", since no meaningful symbol or message or image is being formed here. The only thing to worry about is that the rice should not be hot, so it will not cook the raw fish and vegetables, ("Bishul", "Cooking") and that one cuts the vegetables into thin small pieces before Shabbos. (avoiding "Tochein", "Grinding")
Otherwise, it would seem that it is fine to make sushi on Shabbos.