And in this case, Van der Meer's experiment showing more "elaborate" brain activity during handwriting was correlational. The conclusion that this leads to better learning outcomes does not follow.

I don't think this is appeal to authority (or the reverse, I guess). Neuroscience is complex, being a scientist/doctor is one signal that the observations here would be based on/reflective of a large body of knowledge. Probably GP's unstated point is that the author isn't a scientist/doctor and they also haven't studied/read up on their own either.

The experts in the sciences usually back their claims up using citations, and can be disproven through the normal methods of disproving citations. not through "vibes"