Value of scientific community statement on brain training
In addition to finding what the truth is, an (often underserved) responsibility of the scientific community is to get that truth out to where people can use it.  I wouldn't be surprised if systematic reviews and meta-analyses like you mention have already been conducted.  Those, however, will probably be effectively inaccessible to the general public, from some combination of where they are published and how they must be written.  Public statements in plain language like the one linked are important for getting the scientific consensus out where the public can use it to avoid wasting their time and money on products that don't work.