Is Okular doing the right thing?

The word "enforce" is in general somewhat ambiguous about to what extent the object is absolutely obtained (see the variation in meanings given by a dictionary), but the post to which JoeF replies already makes clear (in more than one paragraph) that its author is aware that the software does not absolutely prevent copying.

(So the answer to the question “how ... does that enforce ...” is: it “urges” or “causes” (to quote one dictionary) that result both by informing the user that an author of the document has requested that the text not be copied or printed, and by requiring the user to go to extra effort to copy or print the document.)

The question under discussion is not whether copying is absolutely prevented (that question has already been answered both in the original article and in the post to which JoeF replies), but whether the software is effective in reducing costly mistakes, and whether there are any practical steps we can take to improve the tradeoff of preventing rare but costly mistakes against the cost of making it less convenient to copy when it is appropriate to copy.

(In this case, without yet having read the discussion in the bug report, I'd suggest that the dialog box could be improved by changing the text to ‘An author of the document has requested not to copy text from this document.’, and going on to inform the user how they can nevertheless copy from the document. Adding a button to the dialog box would be something to consider, though the trade-off is that users don't then get much chance to think about the reason for the request. Again, I haven't read the discussion in the bug report.)

And more generally under discussion is how one might handle other cases where there is some desire to hinder some user actions ("limit freedom") to prevent harm or achieve some other desirable result.