From what I understand, the idea is that you don't use a bundler, but let the browser download all the modules that your app needs - hence some features like the "differential serving" (a.k.a. polyfills added if necessary with UA sniffing).

The primary purpose is to serve packages as JS files in ES module syntax that browsers can use natively: https://www.pika.dev/blog/pika-web-a-future-without-webpack

No. But browsers with native ES module support will be able to import it. (You dont need a bundler if you target browsers with native support)