Yes, it is illogical to assume that Behab as we know it existed in the times of the Beis Hamikdosh. My suggestion is based on two assumptions:
One is that the underpinnings of the concept of Behab originated in zman hamikdosh, when olei haregel came to Yerushalayim in large (?) numbers and it was likely that afterwards there was a need for teshuvah for reasons that are known. The second assumption is that in the times of the mikdosh there actually was some sort of tshuvah act or ceremony after Succos and Pesach parallel (but obviously not identical) to the later development of Behab.
If this is true, then this post-chag tshuvah would definitely have been postponed until the olei regel arrived home two weeks or so later. And so it remains today.