I'm pretty sure they've separated the concerns so-to-speak. The Resultset class is just a container - it doesn't know the details of what it contains.
I assume you want to do this so you can save a query? I believe one way around this would be to manually set, or cache the model's meta data. That way you could simply initialize a new instance of the class and access its getModelsMetaData()->getPrimaryKeyAttributes() method.