Preview in MacOS also warns you that black boxes are not really redacting anything, and provides a tool for it.

Maybe it should instead do actual redaction? Or at least ask if that's what the user wants?

I think that's actually hard... because you have to know what the text is you're redacting to get the spacing for the letters right. You want to redact specific words, but keep the rest of the document the same. This means that you need to know the letters (and their order due to kerning).

The best way to redact a PDF is with a printer, a sharpie marker, and a scanner.

No, it's entirely straight forward to simply remove any curves and glyphs that intersect with a region, even if all text was converted to paths; any vector graphics editor can do it.

PDF documents do not reflow.

It's definitely not as simple as you're suggesting. PDF viewers are not vector graphics editors so they'd have to implement the whole intersection algorithms, and even though PDFs don't reflow they can still have text so now you need to figure out if you delete "is" from "this is hard" you need to calculate where "hard" is.

Definitely doable but hardly straight forward.

I think their point might be that some PDF editors had this functionality for years now ?

I don't think so. Which PDF editors have this feature?

Acrobat (not Reader), Foxit, others I tried. It's not impossible, although the result may vary.

PDFs, inheriting from PostScript, do have the concept of fonts and text strings, which can be edited. They're not just a soup of vector shapes.

Better to draw boxes, print, then scan. I reckon it will somehow be possible to see through the sharpie -- for example change in shine.