Sure thing. In file diffamp2, under "design parameters", see that Rinc is defined as common mode input resistance, between either input and ground. In file diffamp3, under "design equations", design equation 6, see that Rinc equals the average of the input resistors.
Now if it had defined Rinc as the input resistance of both inputs tied together and ground, it might make sense, but that is not how it is worded. Even then, it would be the two resistors in parallel, not the average of them.