I’m not sure yet, but it sounds like Lemmy is within the scope of the legislation.

Key features of the SMMA include:

• Requires age‐restricted social media platforms to take reasonable steps to prevent Australians under 16 years old from having accounts (the minimum age obligation),

Under the SMMA, a platform is an ‘age‐restricted social media platform’ if (section 63C):

• the sole purpose, or a significant purpose, of the service is to enable online social interaction between 2 or more end‐users,
• the service allows end‐users to link to, or interact with, some or all of the other end‐users,
• the service allows end‐users to post material on the service, and
• it meets such other conditions (if any) as are set out in the legislative rules