Gotcha. Thank you for the clarification.
When you deleted them from your Library that is what would have removed them from all Playlists (since they are no longer in your Library to be in your Playlists). Since you chose to keep the files they are likely going to be easier to add back into your Library.
Import music from a computer to Music on Mac - Apple Support
"Import music from a computer to Music on Mac
If you have songs or music video files on your computer, you can import them into Music so they appear in your music library.
- In the Music app on your Mac, choose File > Add To Library or File > Import.
- If you selected “Copy files to Music Media folder when adding to library” in Files settings, you see File > Import.
- Locate a file or folder, then click Open.
- If you add a folder, all the files it contains are added to your library.
Tip: You can also drag a file or folder from the Finder to the Music window to import it.
By default, Music places a copy of each audio file in the Music folder (Home/Music/Music) and leaves the original file in the current location. For information about adding an alias to the file (instead of creating a copy), see Change where your music files are stored.
When you import an item by dragging it to the Music window or choosing File > Add To Library or File > Import, a reference (or “pointer”) to the item—not the item itself—is placed in the Music folder. You might not be able to play the song if you move it."
For future reference this is the option I believe you intended to use when deleting the songs from the Playlist they didn't belong in.
Create, edit, and delete playlists in Music on Mac - Apple Support
"Remove an item: Select the item and press the Delete key.
- If the deleted song is already in your library, removing it from a playlist doesn’t remove it from your library or storage device."
I hope this helps.