You are correct. This is not a Esri geodatabase, it is a SQL Server database using the geometry data type.
I wish I could share some screen shots of what I'm seeing, but unfortunately content sensitivity will not allow it. I think if you follow these steps, you'll see what I'm talking about.
- Add a table from SQL Server that has geometry within it (as well as a GUID field) to ArcMap
- Right click on the layer you just added>Joins and Relates>Join
- Choose the GUID field for this input layer
- For table to join to, navigate to another table with the same database that has a GUID that you'd like to join to
- Choose a GUID field to base the join on
You should see that the GUID fields are not listed as an option to choose from in the drop down. Only Text fields show up as options.
Now try the same join except this time add the table directly to Arcmap so that it's listed in the table of contents.
- Add a table from SQL Server that has geometry within it (as well as a GUID field) to ArcMap
- Add another table to ArcMap that has a GUID field
- Right click on the layer you just added>Joins and Relates>Join
- Choose the GUID field for this input layer
- For table to join to, select the table from step 2 in the drop down
- Choose a GUID field to base the join on
You should see the GUID field/s as an option to choose from
I'm hoping this workflow makes sense and that you can repeat it on your end. If not let me know.