Can in-memory tables be added to a database diagram
You can't add OLTP object in Database Diagram, not in even in SQL Server 2019.
I thought there should be a way to modify [definition] column in [dbo].[sysdiagrams] but it is HexString of unknown file type. (I tried many formats but its obviously an internal Microsoft type)
Unfortunately, there is no reference to mention that is a not-supported feature. (I send a comment to this page )
OLTP is not supported for database diagram. You do not have access to in-memory tables in the diagram because the diagram does not recognize the essence of a in-memory tables as a table, in fact SQL Server generates a DLL for each created Memory-Optimized Table Type that includes the functions required for accessing the indexes and retrieving data from the related Memory-Optimized Table Variable
If you run the SQL Profiler tool you'll see there is a column name IsMemoryOptimized in the table data result set that is returned for the memory-optimized table. I think since the Database Diagrams functionality is older (since mssql 2000) and not updated regularly it does not support viewing the newer memory-optimized tables.