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