It is a “research note”. It might not pan out, and you might say it doesn’t deserve the attention on the internet. But it did suggest something that resembles of compression, just no experiment done for that.

Isn't the delta fed to an arithmetic coder?

Isn't that nitpicking? It's a smaller representation of the data, if you have a certain appetite for decompression time. It could conceivably be worth it. I think it would make a great level 2 cache for older chats.