Comparing this to API access feels odd to me. Opencode does not magically convert your subscription into API usage. It is just an alternative to the official CLI. It has a web UI, smoother UX, and less flickering. Nothing groundbreaking, but it is pretty annoying that even something as simple as tagging files with @ is still so laggy.
How do you think Claude Code and OpenCode communicate with Anthropic? Through the API. Maybe it's accessed slightly differently for subscription users. Anthropic is saying you can't use the API via a subscription anywhere you want. Only Claude Code can use it that way.