As far as I am concerned, you indeed can edit and make flashcards of your highlights. In your PDF, you click on the document symbol in the top left corner, that will alternate your view to a document associated to the PDF. You can then edit the content of the PDF highlights and create flashcards there regularly.
As I understand, RemNote creates a document associated to the PDF. All your text highlights turn into rems in this document, ordered automatically by page. These Rems have a pin to direct them to the correct place in the PDF view. And they are just Rems, as all others, with the same capabilities.
Here, I created a cloze flashcard of the first Rem; and edited the image highlight of the second Rem (inserting a LaTeX formula).