This is false. Although it's true that React makes the most sense when each individual component is all React, it's easy to embed small React components into a non-React application, and you can also wrap small jQuery plugins (like select2, for instance) in React without too much work.

At Khan Academy we've decided to move to React for new JS development but we've had a lot of success moving to it incrementally and building new pieces in React without having to touch the old ones.