Stuck out to me as well—author uses it only once apart from the title, and it's in scare quotes. Are they calling attention to the fact that it's not the usual form of the word, but then failing to explain why that's important to the subject of the post?
In any case, TIL that although "quit" is most common for past tense/past participle, "quitted" is sometimes included in dictionaries as an alternative.
The author is French, the usage of quitted is more likely a mistake outright. As for the quoted version it's explained next to it, he's quitting professionally but likely will continue as a hobby, in French you'd use quotes to highlight the fact it's not to be taken literally.
"To quit" is an irregular verb in English, the past tense is just quit instead of quitted. So "I quit" can be either present or past tense, but from context it would be clear that "I quit infosec" is past tense.
It is valid in English to use quitted in this manner, but it does look and sound odd. Most dictionaries list quitted as an alternative simple past/past participle of to quit, but admittedly it's uncommon to see it in modern English. Usually quitted is used in the sense departed or left (following French usage), which, while perhaps archaic, is perfectly valid in English as well.
In any case, TIL that although "quit" is most common for past tense/past participle, "quitted" is sometimes included in dictionaries as an alternative.