Obviously it's doable in Java, but I'm hard-pressed to imagine a Java developer would think of such a thing.
My best guess at what happened, based on the shape of the API, is that they implemented it by taking their pre-existing domain model, which had a fairly deep subclassing hierarchy, liberally sprinkled some annotations from com.fasterxml.jackson.annotation, and dumped the result straight onto the wire.
You could absolutely do an object-oriented codebase where two different subclasses have fields with the same name and different types, and, depending on how you structure your code, it might not be too painful. And it's fairly easy to imagine someone serializing a structure like that to JSON without ever meditating on the fact that JSON won't retain the all the type information.
Ironically, the end result was an API that is nigh-impossible to consume from Java. I ended up writing a façade in Python.
I've also seen little bits of this happen in the internal APIs at my current company. Also a Java shop, also a result of trying to directly connect an internal object model to the API. I've never seen it done in an API implemented in a dynamic or functional language.
Mainly because I work with classes that are serialized both to and from JSON, rather than having Jackson annotations added much later.
I can see it now. Horrifying! I'm sorry for your troubles.
The code-first approach only works well when you're doing something self-contained. Which, an API, almost by definition, is not.
It's ugly and verbose but there is no need to use empty interface.
_sigh_
This is horrible.