He's asking about base-emoji, not Base65536.

>The emojis used are in emojis.json. There are 843 emojis there, but the converter reads sequences of 8 bits at a time, and so only maps the value to the first 256 of them.

One byte per emoji means 140 bytes per tweet. Since it's just a joke format they're not trying to be space efficient.

Using all 843 emojis would result in 170 bytes per tweet (ln 843 / ln 2 / 8 * 140).