Indeed in BoostNote they store the notes as JSON files with an ID as name, so not more editable than if it was in a database. That's a note:
{"_id":"note:0Rxbbhzib","title":"dfqdsfqdsfds","content":"one line\ntwo line","tags":[],"folderPathname":"/aaaaaaaaa","data":{},"createdAt":"2021-05-07T12:12:49.050Z","updatedAt":"2021-05-07T12:17:28.062Z","trashed":false,"_rev":"YII8y4KkE"}
Probably they've started with plain text files but realised it's a bad idea and went for a database in the end. It seems that's what most note taking apps do, start with plain text files, then when they realise all the problems it causes and how difficult it is to add any new feature, they switch to a proper database.
In fact I feel it's a bit of a waste of time to build an app around plain text files. Because why bother? If they really are plain text files just use VSCode or some editor to manage them, then there's no need for a specialised app.