it’s a causal writing style that has become popular online in the last decade or so. don’t overthink it

It's usual in chats, not for formal writing like officially announcing your resignation. At least that's my impression.

Maybe that's why it's used ? Twitter is just addressed to the public, so it's not official in any way, it's just communication. The lowercase casual style is only used to drive empathy probably.

To be fair, that's not a very formal resignation. He just... quit.

This is the sector of the economy that ipos in cargo shorts I don't think formal writing would be expected or appreciated

Yes it likely is very likely he had absolutely no idea and rushed something out

Reminds me of how rich YouTubers make apology videos. No makeup (but yes makeup), no script (but yes script)... make an effort to seem effortless

I've seen lowercase wording used by people who don't want to put effort into something. Personally, I consider it a sign of dismissal towards the audience. If you see this coming from someone, it might be an indication they don't case. This is based purely on personal experience.