As I understand it, Buffer inherits from BufferInterface, where it uses TransformRegistration (also defined in the same file) to check if a type is supported, in this case PointStamped. Specifically, it uses the method get to check if that type is supported. Since the output you posted is an empty dictionary, I'm assuming there are no supported types.
They also define an add method that requires the type and a callback (no idea where to get this callback, though).