Sorry to bump on this - but we’re trying to fight an ongoing spam attack on our product’s discussion forum 
One of the solutions we’re considering is upgrading new signups to TL1 automatically if they have previously signed up to our product (on an external site), and then to severely restrict all TL0 users.
Based on the current usage pattern, our legitimate users would always start at TL1 whereas spammers would start at TL0. So, being able to rate limit topics for TL0 users specifically would be helpful.
It looks such a setting does not exist. The blog suggests that it should. Hence, I’m wondering if this is a bug. If this is intentional, then please consider my post as a feature request 