I've found that the 'Is (where clause)' is fairly restricted and not very well documented - sometimes you can use it like a regular SQL statement and sometimes you can't. I know that you can do what you want if you switch it to use a SQL statement instead of the rules editor and the only helpful note I have is that you need to use the fully-resolved server name, like this:
<server name>.dwdata.dbo.<database table name>
Here's an example from one of my development VMs that references a DocuWare file cabinet called Employee Files. This is filtering the available documents to those that have the same Facility as what's listed for the currently logged-in user (the Employee Files cabinet has fields called First Name and Last Name):
[FACILITY] = (SELECT Facility FROM [desktop-d89t57g].dwdata.dbo.EMPLOYEE_FILES as EMPF WHERE EMPF.[FIRST_NAME] + ' ' + EMPF.[LAST_NAME] = CURRENTUSERLONGNAME())
Of course, this will only work in an on-premise system because you can't use SQL statements in a cloud system.