iMessage is E2E-encrypted; the only scanning of the content of the network a state adversary can do is by getting Apple to implement it for them, into the iMessage client.

And Apple until now have had the excuse for not doing so, that they have no mechanism built into iMessage that would or could scan for such things, nor (as the excuse would go) is such a technology likely to be either “feasible” given the E2E-encrypted nature of the communications, or “palatable” to their audience (i.e. an argument like “people buy the iDevices in part because of advertised privacy benefits, so such state-level intrusions would lead to nobody buying iDevices, so nobody would be caught by this scheme anyway, so all you’re really doing by asking us to implement it is trying to destroy our company’s share price, probably to help your domestic phone market—and our government wouldn’t take kindly to that kind of market interference. So if you don’t want to be slapped with even more Huawei-like trade sanctions, kindly go away.”)

The implementation of a CSAM scanner is a clear feasibility proof for the set of technologically-equivalent capabilities (e.g. dissident-material scanning.) The fact that Apple have implemented it, and that their users have accepted it and continued to use their iDevices, means Apple has no leg to stand on when denying governments the implementation of technological enforcement for “banning political dissident materials.”