There is nothing preventing someone from decoding this message. Sure it's a bit of a hassle to reverse-engineer the isomorphic function but there is nothing secure about this. The cheat codes are not encrypted, they are obfuscated.
Wikipedia defines encryption as "the process of encoding a message or information in such a way that only authorized parties can access it and those who are not authorized cannot."
That's a poor definition. If I read someone's GPG password off a Post-It on their desk, I can decrypt any information encrypted with it, but I'm not authorized to do so.
A better definition would distinguish between those in possession of both the algorithm and all its inputs, and those who aren't in possession, and rather than saying they "cannot" access the encoded information, it would say that access is impractical or at least cumbersome.
By that better definition, it's still encryption, because it made it cumbersome for someone to access the codes without understanding the encryption method.
> because it made it cumbersome for someone to access the codes without understanding the encryption method.
I disagree. This is security through obscurity. You are simply transforming data, byte-by-byte without any key. This is encoding data using an isomorphic function. By you definition, GZIP is encryption.
I've only read the beginning of that Tweetstorm, but it looks like a transposition cipher (which is considered a form of encryption). Even ROT13 is encryption, despite being trivial to break; a diary lock which can be picked with a paperclip is still a lock, it's just a bad one.
Wikipedia defines encryption as "the process of encoding a message or information in such a way that only authorized parties can access it and those who are not authorized cannot."