A TSB doesn't guarantee a fix, it's only an attempt. I can almost bet money that the new fluid and additive are a combination to increase Cf for the synchronizers to grab. My take on it is a problem with clutch mechanism. The concentric slave cylinder design may look like a clever design, but in the end, it's more problematic than the design it replaced.
Later years GM cars have this problem; two Z06s, a CTS-V and a Solstice GXP, they're all the same; clutch fluid goes dark quickly and gear engagement becomes more difficult since the clutch doesnt fully disengage. My Boss 302 had a different problem, as engine speed increased, clutch pedal effort went progressively heavier until about 6500-7000 rpm when the effort became very heavy. I think it's a problem with clutch mechanism, not transmission. The problem exists in later years Nissan 350Z after the clutch design changed to concentric slave cylinder; the difference is that the number of occurence is less.