Looks to me like these "watermarks" are embedded in monetary numbers, acronyms etc. Maybe to stop them breaking into different lines?
It's not watermarks, it's just scraped typography.
https://en.wikipedia.org/wiki/Non-breaking_space
Different display apps may "display them identically" but others and typesetters/printing apps might not.
Still hope HN adds a filter to block any comment with those characters in it :)
It is a very basic watermark technique (text steganography) if it indeed is supposed to be one.
A more advanced one would be a linguistic (grammar-based) one, but I am not going to give any more ideas. :D
I mean, sure, these characters could be used to help estimate the likelihood text was generated (because human writers might be less likely to add proper non-breaking spaces), but I doubt these are watermarks.