Mysql 8 instance with managed password not supported by RDS Proxy

I read that Mysql 8 should be supported by RDS Proxy here, yet I get an error message in AWS Console that it is not for a mysql 8.0.85 instance with managed password by RDS (using secrets manager): https://aws.amazon.com/about-aws/whats-new/2021/10/amazon-rds-proxy-supports-amazon-rds-mysql-version-8-0/

Why does it say "This feature is currently available only for single-writer Aurora databases"? Why can't my mysql 8 instance be supported?