I'd like to jump in and note _why_ this isn't merely ad hominem attack.
A key point of stating that you despise the use of anything but "proper grammar" inherently carries the conviction that the author understands grammar. The statement, in the context of an argument about grammar, is otherwise pointless; it aims to impress upon others that the author's knowledge of grammar is complete and accurate.
If the same author goes on to make grammatical mistakes, they (yes, they, given we are speaking of a hypothetical author, not Ruffhaus specifically; and the gender of the hypothetical author is not known) undermine this claim in their own writing through these mistakes. The assertion of the author's skill is contradicted by the actions of the author, and in turn, it is reasonable to highlight this contradiction due to the central debate being about grammar.
Also, Retillion, gender identity "wokeness" began long after the use of the singular they became a part of proper English grammar; it cannot possibly have happened under political duress unless you mean to suggest that those willing to adjust a language for political aims have developed time machines, and also have incredibly limited imaginations when it comes to employing those time machines.
On a further note, from your description of ad hominem and relying on Schopenhauer as an authority therein, you assert the accuracy of the title of Schopenhauer's work; thus, by describing ad hominem as a strategem listed, without other comment, you assert that it is a valid way to win an argument.