They could, but adding random zero width characters into words would also destroy ever spell checker, giving it away immediately and making sure that even unaware people would filter it. Doing it outside the words would leave them with too few spots to use for proper watermarking.

I think it's far more likely they'll use some kind of pattern in the tokens - that way the watermark will remain even when you don't copypaste it.

But yeah, as said, they will never tell how it's implemented, but it can still be simply subverted.