Is there a way of adding comments in the Inspector’s Comment section without having to first have an annotation inside a document or folder?
Comments always need to be anchored to a specific location in the text.
If you want an “unanchored” comment that pertains to the entire document, use the Document Notes instead.
If you want an “invisible” comment that doesn’t show in the output document, use the Compile settings.
The definition of an unfastened note in Scrivener is the inline annotation. It can be placed anywhere, with or without text, and even in empty documents. The inspector tool lists comments and footnotes that exist within the document. It could be better thought of as a very stylised and feature-focused search result.