Yes, it's more "leaky," though that's not quite the term I'd use. Instead, I'd say there are fewer checks. On the mailing list side of things, you have all the Postfix bells and whistles, which stop a ton of crap, much of it before the message is officially entered into the mail.python.org system. Behind that is a SpamBayes instance to pick up any loose ends.
The Usenet gateway feeds into the system behind everything except the SpamBayes instance. It gets only sporadic attention from me. If I'm not paying attention, stuff which starts to "leak" through doesn't get trained as spam so it can help minimize the chances that later versions of the same crap get through. One thing which never got produced was an easy way for a list moderator to say, "Hey, this got through and it's spam." Sorting through "unsure" messages and retraining automatically using some Mailman/SpamBayes conduit would be a nice addition to the overall system. If you wanted to write software, that's where I'd focus my efforts. Skip -- https://mail.python.org/mailman/listinfo/python-list