So a tree, but worse.

What makes you say it’s worse?

The underlying conceptual structure is still a tree, but now you have to unfold it into a line, with one reply potentially having nothing to do with the one before it (because they're on very different branches).

No, the underlying structure is a DAG. I've already stated this, and that heavy UI affordances should be made to support ease of interaction with this model.

4chan threads are more like DAGs

The graph of 4chan threads are definitely cyclical, since they can include back-references.

To be cyclical they’d need to have elements in reply to a post made after they were created. Including back references is a way of adding more parents, not children.

You can reference posts that haven’t happened yet

You are referring to providing a sequential post ID past the current latest? That is more of a quirk of their naming convention than a product feature. UUID's, for instance, would not support this, and checks could be made in either case to prevent it.