Could you elaborate why an epiphenomenon isn't viable? Being based on grandmother cell theory seems like an advantage.
Similarly, Jeff has a theory that all representations in the brain include spatial representations. Now he claims the grid cell does that. But how would the grid cell have evolved to do provide the spatial coordinates? Could not the spatial coordinates be part of the sparse coding for the entity in question - why do they need the grid cells?
His new theory, if I understand it correctly, is pretty basic. It is simply saying every sparse encoding of sensory inputs is also tied to a spatial encoding by sets of grid cell modules within the same cortical column. For deep learning practitioners, it would be like saying every embedding comes with it's own spatial encoding, or placed into the same embedding space, however you want to look at it.
These spatial encodings are then related to other spatial encodings of other sensory inputs on different cortical column stacks (by displacement cells?), to firstly build up representations of objects, and then, he surmises further, the relationships of these objects with each other (part to whole).
I think the last part is probably what Jeff will need to substantiate a bit more.
I think Numenta is basing their claim on other studies that suggest there are grid cell like signatures in the neocortex, thus leading to the rampant speculation.