The new drives combine traditional platters with iNAND UFS memory to improve speeds and reliability. These drives are not similar to existing hybrid SSHDs since OptiNAND is not used to store user data. Instead, the NAND memory stores metadata from HDD operations and RRO defects, plus it can improve performance by buffering out more data, and also increase reliability allowing 50x more data to be flushed.
https://www.notebookcheck.net/HDDs-still-going-strong-Western-Digital-announces-20-TB-drives-with-OptiNAND-flash-memory.558014.0.html