Common mode input impedance
what am I doing wrong?


Nothing! The datasheet is wrong. (Yeah, that happens, we're humans and all make mistakes).

The common mode impedance is R1//R2 + R7 = R1 / 2 + R7 = 22.5 kohm.

I think the "2" for placing two resistors in parallel (R1 and R2) somehow got attached to R7 in the datasheet resulting in a wrong value.