How is this relevant?

Replying directly to that sort of sniping would be appropriate even if the relevance were limited purely to being a follow up to the personal remark. However, in this case the more significant relevance is:

• shminux has been the dominant player on this thread (and a significant player in most recent threads that are relevant). He has a very clear position that he used this conversation to express.
• Wedrifid refuted a couple of points that shminux tried to make and, if I recall, was one of the people who answered a rhetorical question of shminux's with a literal answer - which represents strong opposition to a point shminux wants to be accepted to the degree of being considered obvious.
• The meaning conveyed by the quoted sentence from shminux is not limited to being specifically about stopping semianonymous from arguing with wedrifid. (After all, I don't want the semianonymous account to argue with me and on the purely denotative level I would agree with that recommendation.)
• Shminux conveying the connotation "wedrifid is irrational and should be ignored" is a way to encourage others to discount any refutations or contradictory opinions that wedrifid may have made or will make to shminux. In fact it is one of the strongest moves shminux has available to him in the context, for the purpose of countering opposition to his beliefs.
• Instead of people taking shminux seriously and discounting anything wedrifid has said in reply I (completely unsurprisingly) think people would be better off taking wedrifid seriously and taking a closer look at just how irrefutable shminux's advocacy actually is. After all, there is more than one reason why shminux would personally find arguing with wedrifid futile. Not all of them require shminux being right.