I think "random projection", as used by the parent, can be things like "low bits of the pixels in this image". If the color depth provides greater resolution than the sensors, then you can expect to have some random data implicit in the image that it would be possible to change in ways that could be provably undetectable.

A tremendous caveat is that when we find ourselves shipping around lots of meaningless random bits, we often quickly reach for lossy compression that doesn't faithfully reproduce those bits, and that can break the scheme.