There are several levels or degrees of editing restriction selectable in Acrobat, so that status message, "You can edit this document", may only refer to whatever limited things the buyer is allowed to do with the document without entering the random authoring password we added to it.
I think the things you were able to do to the file -- such as adding highlights and strikethroughs, perhaps also including adding notes and annotations -- aren't counted as "editing" the file; they may be things any reader is allowed to add onto their copy of any PDF, much like highlighting or writing margin notes in a printed book or paper draft, or they may be permitted by whatever specific editing restriction settings you have in place. "Editing" in a substantial sense would be changing the existing text-copy or layout, adding/removing/resizing images, adding/removing/reordering pages, that sort of thing. Are you able to do any of those things to your stamped copy, and if so, can you actually save changes?
I'll have to ask Development, but it's possible that any security settings in your original file would be replaced by whatever security settings our PDF Stamper can apply, since the stamper is essentially creating a new PDF for each buyer, by opening your original and adding the stamp to it before re-saving the file to issue to the buyer. Of course, if you want to prevent copy-pasting and/or printing from the buyer's stamped copy, that should preferably be done using the &pdf_allow_copy=n&pdf_allow_print=n parameters in the PDF Stamping URL you configure for the product.