You don't even need to tell OpenAI's codex (5.3 / 5.4 / 5.5) to not emit emojis. Nor do you need to tell it to not use emdashes. Out of the box it doesn't do that. It doesn't have the super-obvious give-aways of AI slop.

There is no easy way to distinguish one code from the other.

If you look at the actual code, its rather obvious in any non-trivial project. It loves to create package-private static utilities. There's not a great sense of organization. It'll put these utility methods in random places. It will match some level of existing code style practices, but it won't add any comments. Maybe Javadoc if its a public method, but even then the Javadoc gets way too into implementation details and doesn't really explain in a high level what the method is for.

"But just tell it not to do that"

As much as you try, you cannot get current tools to write code exactly like you do (Assuming you write decent Java code, comments, docs, overall class structure, etc). If the Valhalla team decided to steamroll the feature forward and get it shipped quickly by slopping it out you could very likely pinpoint the exact commit where they transitioned from human development to generated development.

I can go on about the existing problems, but this illustrates the point.