I can't really imagine how would this work in practice. While "I'm using industry standart AES encryption" may mean the cypher and the key itself will not be breakable, the bigger issue is how to get the 256b key from the player. Does he expect them to actually figure out and manually input 265b of data? That would be a pretty hefty game design challenge to make something like that possible.

I'm betting there's probably something that generates the key from a vastly smaller player input, i.e what gameobjects you interacted with, in what order, or what did you press/place somwhere. But that also means that the entropy is probably in the bruteforcable range, and once you find the function that decrypts the secrets, it should be pretty easy to find the function that generates the key, and the inputs it takes.

~~The only~~ A solution to keeping data from data-miners I can imagine would require just storing the key on the server - which could generally also be bypassed, since then you probably need a way to request the key, which could be data-mined and faked, so you're back at step one - how to validate requests for the key.

Or just make the secret puzzles so difficult, that they can't be brute-forced and the result really is 256b or more of data. Thinking about it, having specified 256 inputs you either have to make or are red-herrings that shouldn't be interacted with isn't really that much, but then the data-miner can just check the location of each one and filter out the inaccessible, and bruteforce the rest. And if all are accessible, it would make for a really difficult secret to discover properly.