The author says that E2 is "generally unmeasurable". Indeed, the true model will be unknown in most cases.
In my opinion, it is not about using E2 or other alternative measure to R2, but about being aware of the significance and validity of R2 and not blindly assuming that a higher value always means a better fit.
In my opinion, it is not about using E2 or other alternative measure to R2, but about being aware of the significance and validity of R2 and not blindly assuming that a higher value always means a better fit.