Don't worry, he always writes like this.
I've heard it referred to as a "flex," basically doing something stupid to rub it in that you can get away with it.
As someone who hung out on IRC way back in the 1990s (and internet-knew Limor from Adafruit back before her handle was Ladyada) I associate this writing style with the culture of a lot of the hacker-related IRC channels I used to hang out in back then.
Some of the same people from that era did in fact turn out to be tech founders and maybe that's how it got carried over into the Twitter-verse, but it predates that.
Most of us would code shift when writing in other milieux, some weaned ourselves off the habit when our work started interfacing with nonreceptive readers, and a few retained the affectation to make a statement (or an anti-statement!).
It's amusing to see the style resurface in a new generation though. I guess it's no more odd than when 20 year olds unknowingly emulate the dress and mannerisms from when their parents were young. We just smile and recall the age when we thought we were being different too. :)
It goes back before that. There were well known Usenet folks who adhered to the style. The 1970s-and-earlier Arpanet was before my time, but I'm sure it existed then too ;).
I was a bit post-Great Renaming into well post-Eternal September. And we may have followed different groups.
The style arises spontaneously in isolated individuals and groups of course (at least since e. e. cummings!), but it was pervasive-to-universal on IRC and MUDs.
I do wonder how it trickled into there though. The most boring answer is probably the correct one. it was slightly easier to type and kids are naturally flexible.
So there's one.
I'm not sure whether I'm brave enough to search the archives for my own writing. :)
Obviously e.e. is the o.g. :)
Not much later, I remember her hanging out in #hack in the '92-93 timeframe (first as lem0n then later as ladyada).
She was like the "kid" of the channel regulars (which is an extremely relative designation because there were a lot of teenagers just a couple years older than her).
I also remember her going to one of the 2600 meetings at the CambridgeSide Galleria that I went to with morgen and wil wheaton that must have been either in 1993 or very early 1994 (it was definitely prior to the first HOPE in 94). IIRC Limor was being chaperoned by RogueAgent and theora/Sarah Gordon.
H.O.P.E. brings back memories though -- I was very uncertain that that Poland Springs jug would contain the explosives taped to the Clipper chip. I remember wishing I was sitting several rows further back for that moment. :)
I rarely capitalise, since i use punctuation and the need for capitals to break up the flow of barely legible cursive is not there.
what's the point? other than social compliance. these things need purpose to live.
london is still London, even without the capital. i am still me, even if I'm not overstating my own self-importance. this sentence still had a clear beginning, even if i didn't highlight it with a T.
capitals are a leftover of cursive.