That article is a bit misleading. The ROM does not modify dumped data in any way. No ROM can do that, because if it did, the computer using it would have to process the ROM contents before they could be used. The ROM can't tell whether it's inside a computer or on a chip programmer when it's being read.

All that article is saying is that they installed an image in the ROM analogous to a watermark image in a photo, so that if someone just copied the ROM directly, that image would be accessible in the copied machines.