Very interesting work! I think it is very well-laid out. I like that they define everything so carefully. Some comments:
We argue that consciousness originally developed as part of the episodic memory system—quite likely the part needed to accomplish that flexible recombining of information.
I think that to demonstrate this one would need to show that having a conscious experience is required to achieve such flexibility. To show that, the 'hard problem' needs to be solved.
In the limitations section, they state:
We wish to clearly state that we are well aware that our so-called explanations of the subjective experience of consciousness do not even begin to get at the hard problem of consciousness—how a collection of neurons and supporting brain tissue produces subjective experience. We are, however, hopeful that our slightly increased understanding of the phenomenology of subjective experience of consciousness can help other researchers look in the right locations and do the right experiments to tackle the hard problem. Our suggestion to them is to focus on the conscious memory system.
But I think that this they are under-stating the severity of this problem. Understanding the hard problem is fundamentally essential to back-up their claim, because the solution to the hard problem tells us the conditions under which conscious experience arises. If we don't know this, then we can't know whether our memory system HAD to give rise to consciousness in order to achieve what it does. What they propose gives us more tools to work on the 'easy problems' of consciousness, but they do sell their hypothesis in the context of trying to solve the hard problem, for example when stating:
Again, we hope that this discussion of what we consider to be key brain regions and structures will help others to dive deeper and achieve a more complete understanding of the hard problem.
And I also hope that it does, because solving the hard problem during my lifetime would be super cool. I just don't see how any of the experiments proposed will move us closer.