jason,
With Pro 1 (Prerelease), it looks that it does not allow to join directly with Excel database like desktop.
I guess, it is not bug or limitation. Maybe, it is a part of workscope for Pro to ensure the integrity of data sources in Pro..
To solve this need in Pro, I would like to do:
1. Create a empty table in GDB;
2. Copy records from Excel into the above table in GDB;
3. Join in Pro to this new table
To finish those steps, all tools are available in Pro
Add data to an existing table—ArcGIS Pro | ArcGIS for Professionals
An overview of the Joins and Relates toolset—Data Management toolbox | ArcGIS for Professionals