The polyfill.js is a popular open source library to support older browsers. 100K+ sites embed it using the cdn.polyfill.io domain. Notable users are JSTOR, Intuit and World Economic Forum. However, in February this year, a Chinese company bought the domain and the Github account. Since then, this domain was caught injecting malware on mobile devices via any site that embeds cdn.polyfill.io. Any complaints were quickly removed (archive here) from the Github repository.
Sure, the package managers of other languages are super safe
You’re confused. It’s unrelated to package managers, it’s about basic security principles like this: https://developer.mozilla.org/en-US/docs/Web/Security/Subresource_Integrity but JS devs don’t care.