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.