As it does say in the New Features Topic:
QUOTE
Custom rank titles
Users with 1000 posts or more can now have their own custom rank title instead of using the default ones provided. This option is available from 'My Controls' >> 'Edit Profile Information'.
Please remember to check existing topics for your answers before starting new topics.
You need to have over 1000 posts before you can customize your rank title. As long as it is within the forum rules, it doesn't matter what it says.