It seems like the crux of the discovery is the "expansion of repertoire" of miRNA, and the control group they're comparing to is vertebrates as a whole, and not specifically humans. According to some quick scanning on wikipedia, the importance of miRNA in neural tissues in mice has been shown by knocking out those genes.

I wonder if the implication is that the massive complexity seen in vertebrate neural tissue is dependent on the ability of RNA to edit itself and somehow enable cognitive capabilities far beyond those encoded in the un-edited DNA?

I'm just an enthusiastic layman so take all what I've written with a pail of salt.