Apologizing for replying late to an email is common practice between colleagues in business, especially if your late reply has blocked that person from doing their job. I don't know who this Ploum guy is, or why he has a nickname, or why French Wikipedia seems to think he's a noteworthy software developer, but I hope for France's sake he isn't actually as influential as he thinks, because this could be disastrous for French-English business communication.