NDB Cluster 8.0.23 contains a new thread model that makes it possible to grow and shrink size of data nodes. This is default in RonDB, optional in NDB. Also RonDB has fully automated the memory configuration such that the user needs only specify which VM instance type he wants and the number of VMs. Plus a number of bug fixes, improvement to monitoring and some more.
Another metaphorical way of describing the difference is that NDB is high-end PC whereas RonDB is a powerful Mac. NDB can be configured in any number of ways whereas RonDB does the configuration automatically and the user will always get access to the most advanced features of NDB.
RonDB is a managed version of NDB in the cloud, and we have some optimizations for its use as a Feature Store for ML.
We call RonDB a distribution of NDB.