Yes, I can't find a 30%-slowdown number either.

I'll add to this: the referenced trial occurred over 8 weeks, so even if we stipulate that the improvements in cognition (which are dubious, as tgv points out in this comment: https://news.ycombinator.com/item?id=48347906) are due to treatment rather than some other effect, we don't know that the effect is disease-modifying as opposed to symptomatic. As with acetylcholinesterase inhibitors, it may just be having a cognition-enhancing effect which, nevertheless, does not alter the underlying disease trajectory (i.e. just shifting the declining trajectory up vertically by a constant amount), and might revert shortly after discontinuing use of the drug.

A controlled trial, over a much longer duration, and ideally with a wash-out period, would be necessary to identify a disease-modifying effect.