ExtReact is a separate product with its own license and pricing. You can buy ExtReact to add components to your React apps. ExtReact Std is $695 per developer per year and ExtReact Premium is $1,195, both include Support, and you must be current on your subscription to develop applications. Rights to distribute and use applications are perpetual.
Sencha customers who already have an Ext JS license can use Ext JS components in React simply by using the extjs-reactor npm package on GitHub (which is governed by the MIT license).