The author starts by telling how he, as a child, confused the directions of "write a poem" and "write down a poem". He then doubles down on this mistake, rather than concluding that English is weird.
The author conflates legality and morality, stating that any group that establishes extralegal moral norms needs to reexamine its assumptions. The author implicitly and incorrectly assumes that laws are the source of morals, rather than existing to incentive moral behavior.
The author fails to consider the effect of reputation. Reputation is a heuristic that is used to determine how likely something is, given how trustworthy the speaker is on the subject. If a researcher in an esoteric field, having contributed several results to the field, suggests a new way to interpret the field, that suggests that the idea has already been reasonably vetted. A crank email, on the other hand, may present similarly novel ideas, but they have not been vetted. Plagiarism inappropriately assigns reputation, subverting this heuristic.
The author's metaphor of the apple tree has some rather unfortunate implications. By the logic given, a person should plant apple trees farther from the property line, so that all apples fall well away from public roads. In scientific discovery, this would mean hiding one's findings, refusing to publish other than to state that a discovery has been made. This was the method that was in use through the 16th century, and resulted in much slower rate of progress. Read, for instance, the multiple discoveries of the Cardano Formula.
In short, the author's case that plagiarism should be abandoned as a concept is severely flawed.