Even though avatar originals are stored on S3 there is still a bunch of logic that needs to happen app side to decide on which size to serve and so on.
Meta uses S3 for uploads and avatars are served from CDNs, we do that by defining the cdn url.
Notice how your avatar is served from the CDN here?
https://global.discourse-cdn.com/meta/user_avatar/meta.discourse.org/jongnam/90/52330_1.png