A federated platform seems to be a big overkill for this use case. Sorry, but that sounds a bit like the time where everyone tried to solve everything with blockchains.
You could simply export and import playlist with a file that can be shared. Companys simply don't want it to be that easy to switch to other services because its a huge selling point that you got all your playlists in their system.
Additionally I don't think that it would be that easy to identify songs between systems because I don't think that there is a unique identifier like the ISBN for books.
A federated platform seems to be a big overkill for this use case. Sorry, but that sounds a bit like the time where everyone tried to solve everything with blockchains. You could simply export and import playlist with a file that can be shared. Companys simply don't want it to be that easy to switch to other services because its a huge selling point that you got all your playlists in their system. Additionally I don't think that it would be that easy to identify songs between systems because I don't think that there is a unique identifier like the ISBN for books.