Let me rephrase my current understanding of this to avoid propagating more speculation: I see no evidence that assist abilities affect the entire team (DMM or Nutaku version). That is not evidence that they DON'T. 8% is such a small buff, and the random modifiers on ability damage are so large (not to mention all the potential confounding variables), that reliably spotting such a small effect statistically is actually pretty difficult and would require some dedicated data collection. I'm not so interested in this that I'd leap to do such a study--not when the only basis for an effect is hearsay.
It's sort of like one person saying, "5 Stacks or more of Snatch decrease damage dealt by mobs by 1%!" There's no basis for such a claim, but I can't disprove it without an inordinate amount of effort. Does that mean we should believe it? No!