Well the article was actually mainly presented from the point of view of a commenter (as opposed to blog author). Notice all three points were about why he didn't like to comment via FB (vs. not liking to receive comments via FB). It was "I don't like commenting on FB, so you probably shouldn't use it" vs. "I have experienced less traffic when using FB comments, and thus am extrapolating that you might as well".

Thus, despite not visiting your site, he can still have an opinion about not liking to potentially comment on it if it contains FB comments. This is not some weird generalization or overstepping his bounds, its the same as someone saying "I don't like sites that have ads so think twice about littering your site with them" and then someone else responding "BUT how do you know if you've never seen my site?!".