All the examples of non-breaking spaces that they showed were arguably places where someone nicely typesetting might well do the same thing. For example, in "FY 2025", or "$8.7 billion". (I've even done this a lot myself in the past.) I wouldn't call this a watermark, but more a sign of likely copy&paste, if students' word processors weren't currently doing that.

A "watermark" that invisibly identifies the text origin using Unicode tricks sounds possible.

And maybe you could do some things with statistical patterns.

Or you could, as some have done in the past, is to stego the identifying information in a way that's hard to spot but can't be denied later (e.g., the first letter of each word clearly spells out "john smith is a cheater who copied this from chatgpt").