Thanks Stephen, I reached out to the developers and received this response
The images are sent to S3 as soon as they are uploaded to the Media Library, whether that’s directly with “Add New” or via “Add Media” when editing a post. So while drafting a post any media should be showing S3 URLs.
If that is correct, but when you publish your post it still sends content with local URLs to Discourse, then it’s likely that whatever plugin you’re using for that is not properly processing “the_content” via WP’s filters. The following guide may be of help.
Filtering URLs in Custom Content
Can anyone confirm that the discourse plugin is using the_content?