It is of course true that arguments should be evaluated on their own merits. However, we rarely have time to do that, and so we trust authoritative thinkers based on their record of producing good reasoning. The commenter who quoted aristotle invoked his authority to lend credibility to the quoted statement. You would have discarded this statement unless it was shown as a quote of airstotle. The next commenter reduced the authority of Aristotle by calling his record of good reasoning into question.

When appeal to authority is invoked, it is perfectly reasonable to question the validity of said authority.