It could instead imply "if the browser didn't ship with a version that satisfies my requirements, use the polyfill"

if they want these libraries to ever be used as default they need to have a stable api, so it needs versioning and the ability to link to specific version or versions in a bracket and the backport of security fixes across releases within a reasonable timeframe

why would anyone develop toward a library that doesn't provide those basic guarantees?

there's also no guarantee that your web app gets deployed to an up to date browser, so it's either you can specify which api level your app wants or we're all back to specify which browser version an app runs onto.