Carrying within 2000 amos (Eruvin 42a)

The gemara quotes a machlokes amoraim in the case of someone who camped out in a large valley with no walls and on Shabbos some people built a wall around the entire valley. He can definitely only walk 2000 amos from where he started but the question is if he can carry. Normally you can carry even if the mechitzos were only put up on Shabbos.
Rav Nachman in the name of Shmuel: This is no different in a regular case so he can carry throughout the valley "al yedei zerika" even though he can only walk 2000 amos.
Rashi: He can carry regularly in the first 2000 amos and he can only throw beyond the 2000 amos because he's not allowed to walk there.
Tosafos: He can't carry anywhere in the bika because it's open l'makom h'asur lo but he can throw throughout the bika even beyond 2000.
Rav Huna:He can only carry the 4 amos. The fence doesn't help because we're afraid that he might follow the object he's throwing. He can't even throw within the first 2000 amos because it's open l'makom h'asur lo.

Then the gemara quotes Rav Chiya bar Rav (might have been named after his great uncle) who says that you can carry within 2000 amos. The gemara then questions that doesn't follow either opinion so how could he say that. The gemara's question is a little difficult to understand because why should he be forced into agreeing with one of the other opinions. He was a contemporary. They all lived the generation after Rav and Shmuel so he should have been able to argue. Rashi therefore explains the gemara's question wasn't that he can't argue but that there is no svara to split the psak. He seems to take a compromise shita which is very difficult to understand.

According to Tosafos above, I think that it's clear that Rav Chiya bar Rav's shita as written originally doesn't make sense. You can never do more within the first 2000 than you can do beyond because there is no heker separating them. Rav Nachman holds you can throw throughout and Rav Huna says you can't throw anywhere. It doesn't make sense to suggest that you can carry only 2000 amos.

However, according to Rashi it's a little harder to understand why the gemara finds Rav Chiya bar Rav's shita impossible to understand. All he's saying is that you can carry within the first 2000 amos but not beyond. That's not so different than Rav Nachman who basically says that but he says you can throw beyond that point. Obviously the gemara assumes that's not considered a makom asur since you can throw there. If you can't even throw there then there's no way you should be allowed to carry within 2000 amos. I can understand the svara but Rav Chiya's opinion as written doesn't seem so crazy either. I would think it would be possible that he believes that really m'ikar hadin you could carry throughout the valley. The only reason you can't carry past there is because you might walk there and we're not going to forbid you from carrying within 2000 just because of that gezeira. I guess the gemara doesn't like that svara - if you can't carry (or throw) there for any reason then that's considered a makom asur.