I've often heard that but I think it's more correct to say that both parties may think it's beneficial to them. Ultimately it is a zero-sum situation where it benefits one party more. When one party decides it's getting the short end of the stick and not likely to regain the long end they'll have plenty of incentive to oppose it.

The non-zero sum benefit of not having to compete in as many districts can't outweigh the harm of being relegated to the minority party. Surely a party would rather compete and win than not compete and lose.

It is a zero sum game so that statement cannot be true unless you mean over time and in different places. If that is the case then it is a great argument for coming up with a set of rules beforehand that both parties agree is fair.

I agree with your comment; but I do have to point out the comment you're replying to specifically conceives a political landscape with more than two parties, that can indeed be considered as zero sum as well, yet two major parties can feasibly agree on something at a net loss for others.