I don't know where you got the idea that nation states are the only ones who use cryptography. Plenty of activists, along with other non-state actors do so all the time.

Plenty of cryptography is also designed by individuals not in the service of any nation state (as far as we know, anyway). In fact, some argue that such encryption is more trustworthy than encryption developed by nation states themselves.

"Also, what makes you think activists have the technical expertise available to know what the "most secure techniques available" are and what methods have been broken?"

I can't speak for any and all activists. It's really up to them to acquire such expertise or get advice from people who have such expertise.

That said, the problem here is no different from figuring out which encryption to use. So your criticism applies equally to encryption as it does to steganography.