This is a fair response and I think would be a good top level comment, you don't need a meat shield. The reason the parent was downvoted was that it isn't criticism, it's just a declaration that they didn't read the article.

There are a lot of citations in the paper backing up the idea that handwriting promotes learning, though (or at least spelling, memory, recall, and letter recognition/understanding). It reads to me like an attempt to explain an observed phenomenon: "given that handwriting appears to promote learning, what could be a mechanism for that?" It isn't in a vacuum just claiming that "these brain signals clearly prove that learning is happening".

The "general discussion" section also covers why the specific connectivity they looked at might be related to learning.