You're right. Agent mode with select models (no o3 or Opus unfortunately) is available to Copilot Pro users (also provided for free to eligible FOSS developers) on latest VSCode (I hadn't updated it in over 5 months). Per docs, the newly announced Coding Agent (available on the website only) however isn't. And this was the source of my confusion.
https://docs.github.com/en/copilot/about-github-copilot/plan... / https://archive.vn/HxHzc