Where do you see "cross-platform"? The readme clearly says it's web:
OpenUI is a full-stack Generative UI framework: a compact streaming-first language, a React runtime with built-in component libraries [...]
The link submitted is to the front page of their website, which reads: “Cross-platform - One spec renders natively to React, React Native, Vue, and more.”
The link submitted is to the front page of their website, which reads: “Cross-platform - One spec renders natively to React, React Native, Vue, and more.”