I remember reading speculation about that as well. After some googling, it appears only speculation that the Music app would be built using Marzipan/Catalyst.

9to5mac writes in a correcting update that “We’ve now learned that the code for the new Music app for macOS will be based on iTunes, not iOS”.

https://9to5mac.com/2019/04/10/macos-10-15-itunes-standalone...