Interesting.
I wonder how well the dual droplet sizes makes up for the lack of light colour inks. Epson has had variable droplet size for a long time, but I believe they use it for speed rather than gamut, using larger droplets and reducing the print resolution in lower-quality print modes (i.e. at the highest quality setting, every droplet is 3pL). If HP are using variable droplet size even at the highest resolution, they may be achieving a higher ink density (6pL drops in a 2400x1200dpi pattern), although this would likely benefit dark/saturated colours more than the colours that typically benefit from light inks.
Certainly, the black-and-white output should be even better, given that there are now effectively five densities of black instead of four (MK, large and small droplets of PK, large and small droplets of G).
But the really relevant comparison will be the print longevity tests, compared with the Z3200. Going backwards isn't out of the question - just look at the Canon Lucia Pro inks compared with the previous Lucia EX range.
So far, the only result we have that can be used for a direct comparison is on HP Professional Matte Canvas. Unframed, under a bare bulb, Wilhelm gives it a rating of 150 years with the Z3200 and 99 years with the Z6200, and >230 years vs 134 years for dark storage. But the Z6200 isn't the Z9, even if it also uses HP Vivid Photo inks - it uses LM, LC and LG inks, whereas the Z9 doesn't use light inks at all. It would be valuable to know which particular inks/colours are causing the increased fade rates compared with the Z3200 - is it just the light inks, or the entire inkset? On the other thing, the fact that HP is claiming improved gamut with a smaller inkset may not be a good thing in this regard - does this mean that they're using less dense inks, with smaller pigment particles, which will give brighter colours (and more capacity to replace the light inks) at the expense of faster fading?