The setting tells you that Sandevistan cannot increase running speed, because like any other cybernetic augmentation in Cyberpunk, it cannot magically make the rest of your body better. And this explicit, unambiguous statement is never retconned or addressed in any setting material, which means it still stands. Sandevistan is supposed to be the "classic implant" which was so common to 2020 and RED, and doesn't increase movement speed in either of those, despite how RED was being built specifically as a prequel and setup for Cyberpunk 2077.
R Talisorian Games knows how to say "this cyberware increases your MOVE stat." They specifically chose not to say this when statting out Sandevistan in Cyberpunk RED, which is pretty telling, because they did in fact change how a lot of ware worked for RED, such as how implant linear frames and cyberarms behaved. So if the Sandevistan increases speed, then we would have seen it in RED. It acts identically in RED as it does in 2020, and absolutely no source claims that there was some sort of cyberware revolution (or even just a Sandevistan-specific revolution) which lets you run faster while using it.
Your claim that "saying game mechanics is not a defense" is blatantly wrong. Cyberpunk 2077 is a RPG, not a tactical shooter, and it's not a 0451 game, but rather a more conventional RPG-FPS hybrid without 0451 games' hyper-intricate level design. This means that you can have greatly divergent game mechanics and actual gameplay. Alpha Protocol allows Michael Thorton to slow down time, create a glowing blue energy shield around himself that lets him eat a RPG to the chest, punch harder than a gun hits, or turn invisible. None of this exists in the reality of the setting, they are game mechanics which exist to enable the RPG systems and represent levels of capability without extremely complex and time-consuming map design.
Sandevistan slows down your perceptions and also is supposed to make you harder to hit in 2013, CP2020, and RED. This is exactly what it does ingame. The fact that it increases movement speed as well ingame is because if it didn't, it would make melee completely worthless compared to guns (c.f. how Kerenznikov was not very helpful for melee builds even before 2.0 limited it to guns only). The reason that it is primarily a defensive implant for close combat attackers in gameplay is, again, probably because being shot in the dick the moment you peek out of cover by a sniper who has had multiple subjective seconds to draw a bead on your dick is aggressively unfun. Moreover, there are a wide variety of augmentations which do exist in setting but aren't mechanized ingame (like how you can use cyberlegs to run faster or have nanomods to your leg muscles to run faster) and can be conveniently folded into "Sandevistan" in game mechanics. It's hardly inexplicable, nonsense, or incoherent.
What is nonsense though is looking at consistent explicit statements that Sandevistan doesn't increase speed, from primary canon, which have lasted for 30+ years and are backed up by the setting's own metaphysics telling you that cyberware is limited by relatively hard bioscience and thus can't just magically imbue secondary powers on people without a proximate physical cause, and then insisting that some game mechanics and a hyper-stylized anime mean we throw out all these consistent statements and this immense body of evidence.