New Mac's iMovie won't open library created with a "newer version" on old Mac.

I just transferred my files from a 2019 iMac to a 2024 Mac Mini using Migration Assistant. Everything came through fine except that iMovie won't open my movie library because it says it was "created with a newer version of iMovie and will be ignored." I checked the App store to see if there's a newer version of iMovie, and no newer version comes up. My version is 10.4.2. My MacOS version is 15.3.1, as it updated just before the files were transferred.

I made sure I had multiple backups before I did this conversion but I was unprepared for this. How can my new Mac have an older version than my old Mac?