Built bundles are not affected. The service is supposed to figure out which polyfills are required by a particular browser and serve different scripts. Because it’s serving different scripts, the scripts cannot be bundled or secured using SRI. That would defeat the purpose of the service.

Code pulled from GitHub or NPM can be audited and it behaves consistently after it has been copied. If the code has a high reputation and gets incorporated into bundles, the code in the bundles doesn’t change. If the project becomes malicious, only recently created bundles are affected. This code is pulled from polyfill.io every time somebody visits the page and recently polyfill.io has been hijacked to sometimes send malicious code instead. Websites that have been up for years can be affected by this.