Fair on AG-UI, that part is a real difference. But the Chat SDK point is a bit overstated: vercel/chat also has a JSX card system that renders to Block Kit and Adaptive Cards, with button and action handlers and pluggable state backends.

> Chat interfaces are inherently multiplayer.

Both chat and channel sdks support subscribing to threads and can control messages that arrive concurrently. In that regard it feels like this depends more on the logic of the app itself rather than gets defined by the framework/dependency it uses.