That’s not actually missing, they did that and some of the taxa came back but most didn’t. Of course we knew microbiota were hereditary and that if all of a species died there’s no reason they’d come back, but the surprising result was that 3/4 of the measured bacteria taxa died off by the 4th generation low-MAC mice compared to the control high-MAC mice. That’s a huge difference in microbiome diversity, though you’re right that harm wasn’t proven I think it’s fair to say it probably was harmful.
I do think the fiber in Soylent is too low, and that RL’s reasoning of it being a liquid is insufficient. While it may still have more fiber than the standard American diet that’s not a reason to keep it where it is.