Toritoribe's a Japanese who knows Japanese. Sometimes native speakers break the rules. They may have done it for effect. There are some people on Twitter who say おそようございます to each other at night, as a play on おはようございます, but strictly speaking it should be おそうございます. They're doing if for comedic effect, and I assume to preserve the rhythm. The authors of 問題な日本語 also employed this sort of breaking of the rules for effect with the title of their book (should be 問題の日本語), just to be ironical.
Of course, sometimes native speakers just aren't familiar with the rules, because (in this case, at least) they've fallen a bit out of use. As I understand it, while ごとし is used in modern language still, it's on the upper end of the formality (and erudition?) scale, being a remnant of older Japanese. You can see people making mistakes with き at the end of adjectives, as well, just putting them after any adjective because they mistakenly think that all adjectives ended in き, when in fact a good chunk of them ended in ひ. But no one uses that grammar in everyday speech anymore, so if you haven't studied historical spellings (at the least) you wouldn't know which ones ended in き and which ones ended in ひ. Plus き may just sound more classical nowadays, so even if someone did know that an adjective ended in ひ, they may use き anyway knowing it isn't accurate, just to generate that effect (but that's speculation).