No, that’s wrong. See Figure 3 below. It shows an increase of about a third in two specific types of upper-layer cortical neurons. The tripling was in a specific kind of progenitor cell.
No. The effect on neuron number is most definitely not extremely large. I don’t know whether the effect on progenitors would be judged “extremely large” but that’s why the authors looked at neurons and their types, and again, it’s a pretty specific population that tripled. It is flat wrong to conclude that there was a global tripling of cortical neurons or even of cortical precursors.
The authors note that cortical neuron generation is in progress at the stage at which they did their analysis, so this is in some sense a snapshot. At some point the progenitors switch to making glial cells, though.
My read at this point is that we should step back from the overstatement in the title of this thread, from the typical overselling of the press release, and from the red herrings in this very discussion, and see the phenotype as a very interesting quantitative change induced by a single mutation. That change is of course big enough to make an evolutionary difference, in principle, but it’s a quantitative, cell biological, specific change.