there is a very big difference between starting an ephemeral instant message without capitalization, and lacking capitalization throughout a more "persistent" long-term medium

It's not derived from that. Most people don't use IRC anymore and there are basically no open chat platforms left. Most people now use phones to chat, where the keyboard automatically changes case for you after finishing a sentence.

So typing in all lowercase is a choice. I think it's a way to signal "I don't follow norms". It signals membership in a group.