It's interesting to think about why that habit exists. It seems like a low barrier to entry to me. If Carmack had wanted to post to Medium or something he would've had to write and edit the whole essay and get people to come read it.
On Twitter Carmack posts a paragraph as it comes to him. Could do the whole thing at once. Could take days to complete the thread or longer. It could be as long as he wants. One tweet or one thousand. No real expectations of edits. No one would be surprised if it's only a three paragraph thought whereas going to read an essay that might feel a little brief. Nobody expects really profound or serious insight, just the thoughts on top of his head.
Readers also have the same low barrier to entry. No need to go to a separate page or app. Look at the first or first n paragraphs. Scroll by anytime.
The user experience for something like this may not be perfect on Twitter, but I think Twitter has a lot to recommend itself as the appropriate tool for sharing thoughts like this.
Reading Carmack's thought to text style writing (a la Joyce's Ulysses) might be the only thing the get me to wade into the sess pool that is Twitter .... Holy crap, that was the most arrogant, self agrandizing post I've ever made =( I wish I didn't feel that way, but I do =)
I detest it for this reason. Well, that and the fact that it's a closed source service leaching off content on another website and slapping ads on top of it. The only thing worse are those awful video downloader bots.
I have opted out and blocked their bot on Twitter which apparently currently suffices to prevent their scraping. I'm more than happy to point anyone who asks to a text file. God, I miss blogs.