How to apply Thevenin's theorem to the equivalent two-port circuit of receiving antenna?
You are assuming that the current i2 is the same as the current passing through V1 which is not the case. The current passing through V1 is not the same as the one passing through Voc.
You apply the current divider rule to get i2 from i1, when shorting ZL it is multiplying the current by Z12/Z22. That is multiplying the final result by Z22/Z12 and you will get the right answer.