This is the reason I added: "where certain rules (public rules) are applied".
Sure, you can create wherever custom_json you want with any token regardless of the balance, and this will be a valid transaction on Hive. However, certain rules apply on Engine, and they are public rules, then that json will be rejected there.
This transaction is not broadcasted because there is no reason to do that, it is already on Hive.

In any case, I agree with you that Engine can not be considered a blockchain because there is only one node running it, there are no incentives to run more nodes, then it is centralized.