This appears to be the actual, recommended solution https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/example-function-add-index.html
So they want me to use either "Cloudfront Functions" or "Lambda" to do a special redirect rule to add index.html. Except! The redirect rule would be solely name-based. I can't tell it "index.html if it's a directory". It *guesses* it's a directory based on the lack of file extensions. So if I take this approach I will never again serve a file which does not have an extension from this domain.
What. The. Actual, Fuck.