The author doesn’t point to similarities between two things in order to conclude another similarity. Also, the premise is not just superficially related to the conclusion. It assumes the conclusion is true. This isn’t good reasoning, but that doesn’t make the premise irrelevant.

The purported evidence (the idea that a complete understanding of physical aspects of a human action still wouldn’t constitute comprehension of the action) assumes that understanding human actions requires something beyond an understanding of physical aspects.

The premise does not assert that there’s no evidence against the idea that understanding human behavior requires inquiry into nonphysical aspects.

Arguments do not have to mention anything about the author’s awareness of potential counterevidence. Although the existence of counterevidence could hurt an argument, it’s not a flaw to omit whether you’re aware of counterevidence.

The author asks us to “suppose” that we had a complete scientific account of physical aspects. This doesn’t mean the author thinks science actually can provide such an account. We’re just considering this hypothetical for the sake of following a line of reasoning.