Spymarks an application of steganography, not a different name for it.

> On that note, one way we can prevent it is to assert that all our content is byte-for-byte identical with the last known trusted stage of what we have produced

That doesn't help with things like the typical use of SynthID where the spymarking is done by the same process generating the content, so there is never a clean comparator. (It also wouldn't be useful anytime it is inplemented as part of a transformation—compression, etc. —step, for the same reason.)