FCPX AutoDuck creates another file with the extension FCPXML in the same folder as the speaker file.
Double click on this file, Final Cut Pro X will open and ask us in which library to insert this project.
Select the library or create a new one.
What you will get is a new project (a timeline), including the audio file of the speaker and the music one, and all the related key frames:
In this example, a 2 hours long speaker and music files were processed in 15 seconds on a 2012 MacBook Pro.