Perhaps if all the accounts were created at the same time, but none of the Catfish accounts were.
It is still spamming because he is making numerous accounts with only the intent of posting 666 times and then not using them again. The fact that he is doing it purposely could qualify as malicious intent on some sites. As I don't know admin and don't know the full setup of the database, but having one person make 666 accounts with 666 posts each would be 443556 posts, which could lead to quite a bit of space being used just by one user.
You must also consider, that if he is already showing the OCD behaviour of 666 posts then making a new account it would lend to itself that once he gets to Catfish666 and do his final 666 post he will do a new variant on Catfish and start over again with the 666. The fact that he has more than one account could also lend to sock puppet or trolling (also noted by the 666) without being blatantly obvious. You can troll in more ways than one and spamming accounts with a set number of posts is a good way to do so. Either way he is making multiple useless accounts instead of sticking to just one, so he is spamming the user database (which one misconception you pointed out is he doesn't have to do it at one time for it to be spamming, as just a single post about Viagra and linking to some site is spamming the site).
I agree with chrisname, you are definitely over analyzing this. The overhead of creating new accounts is probably pretty minimal and I doubt the admin would care. The number of isn't a good argument because it would take ~12 years posting 100 times a day, and if you look he doesn't (hasn't yet at least) posted nearly that often. Plus his posts are no worse then yours or mine.
I am a programmer, analyzing things is just a part of what I do :P.
Plus his posts are no worse then yours or mine.
Said the content of the posts made no difference. My point was that he has to be careful making multiple accounts, joke or not, as I know of a few admins on other sites that would consider it spam and delete the extra accounts (mainly due to them having policies of only one account per user).