Hard to say with 100% certainty without a human trial, but the short answer is "probably not." This is a situation where a person has three copies of chromosome 21 in every cell. Shutting one copy down would, hypothetically, leave someone with two working copies. I don't think we have any reason to believe that trisomy is masking some other phenomenon that we won't see until a fetus with this treatment applied fully develops into a newborn.

(And that's of course assuming human trials were authorized. Probably not for this treatment in my lifetime, at least not in the US).