I wonder if it’s common for those steganography techniques to have some mechanism for defeating the fairly simple strategy of getting 2 copies of the file from different sources, and looking at the differences between them to expose all the watermarks.

(I’d think you would need sections of watermark that overlap for any 2 or n copies of the data, which may be pretty easy in many cases, though the difference makes detecting the general watermarking strategy massively easier for the un-watermarkers)