Thanks Mattie for taking a look!
I deleted the files, tested local and production (all pushed fine, showing the “basic” editor) and started again. I used yarn (install, build). And I run into the same issue BUT:
I noticed that a package.json is being generated in the plugins folder, and it has only the 2 lines for ckeditor. I copied them over manually to the “main” package.json file (in my case in the “backend” folder of my git) and heroku run with no issues!
Not sure if I did something wrong while following the instructions to end up with 2 package.json files, or if that is ok and there should be a hook between them… but for now copy-pasting the 2 lines seems to have done the trick 