@Nes_Hdr Single user compute uses fine-grained access control to access the tables with RLS/CLM enabled.

There is no specific details about OPTIMIZE being supported in Single user mode. 

Under this doc limitations of FGAC mentions that 

"No support for write or refresh table operations on tables that have row filters or column masks applied.

Specifically, DML operations, such as INSERT, DELETE, UPDATE, REFRESH TABLE, and MERGE, are not supported. You can only read (SELECT) from these tables."
https://docs.databricks.com/en/compute/single-user-fgac.html