The browser, iOS, and Android all use a main thread separate from the thread responsible for scrolling animations.
However, I think it's true that controlling threading in order to perform gnarly work in a separate thread is more ergonomic on mobile than in JS/React.
You'd need to create a Promise that wraps a Web Worker, which would be an unusual thing to use. I don't think most apps need such control over threading in the browser.
If you care about smooth interactions, the browser even in 2026 is not the most straightforward route, nor the route any reasonable small company takes.
However, I think it's true that controlling threading in order to perform gnarly work in a separate thread is more ergonomic on mobile than in JS/React.
You'd need to create a Promise that wraps a Web Worker, which would be an unusual thing to use. I don't think most apps need such control over threading in the browser.