Copyright laws are tricky and may be different in each country. It is particularly messy in in Germany, where most artists are transferred the management of the copyright to the GEMA. But the copyright to a song will always belong to the artist. The artist can grant the use of the song under certain conditions. Look at the licence posted by Limnos and check the items that are not mentioned.
The GarageBand licence is giving us explicit rights for a commercial use of the audio content, and a similar statement is missing from the iMovie licence. So I would be careful. It would be safer to create the soundtrack for your iMovie Project in GarageBand - it has the nicer licence that grants you the use to create a song arrangement for commercial use and distribution: Using royalty-free loops in GarageBand with commercial work - Apple Support
Learn about the usage of royalty-free loops in GarageBand on commercial songs, multimedia presentations, videos, and animations.
The GarageBand software license agreement says:
"GarageBand Software. You may use the Apple and third party audio loop content (Audio Content), contained in or otherwise included with the Apple Software, on a royalty-free basis, to create your own original music compositions or audio projects. You may broadcast and/or distribute your own music compositions or audio projects that were created using the Audio Content, however, individual audio loops may not be commercially or otherwise distributed on a standalone basis, nor may they be repackaged in whole or in part as audio samples, sound effects or music beds."
So don't worry, you can make commercial music with GarageBand, you just can't distribute the loops as loops.
Published Date: January 18, 2017
All iMovie content from the older "iLIfe Sound Effects" and "iMovie Sound Effects" are appearing in my GrarageBand Loop Browser and can be used to create a sound track for a video that has been added to the Movie Track in garageBAnd.