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.
