Have I successfully (permanently) watermarked my PDF?

I've got old physical picture books I've scanned and turned into a PDF. I'm trying to watermark it so that it is permanently irremovable (i.e. it is 'one' with the rest of the page).

Here is my process:

1. Add a comment (stamp) to the document which acts as the watermark
2. and then use the java script "event.target.flattenPages();
3. then finally use "Sanitize Document"  to remove overlapping information.

When I look into the "contents" tab I am only seeing one image (see attachment) and if I delete it both the watermark and rest of the page is deleted ...  which makes me think this worked ... but did it actually? Is the watermark now irremovable (at least easily) from the PDF?