If it is instantaneously zero-sum, then shouldn't it be the same over longer periods too? How can it be anything else? Where would the first non-zero-sum trade come from, and who would be on the opposite side?
For me it is zero-sum (well negative with costs, but we're putting that aside). It doesn't really matter that someone has some other target, hedging etc, besides winning on the trade, ultimately the money has to come from somewhere and so someone is losing or overpaying and someone is winning or underpaying.
Besides that, as pointed out, sometimes the idea is more important than whether it applies to every single instrument. Wyckoff might talk of a composite operator, we might say 'the market' wants to go there etc, or we may talk about strong hands, weak hands. These may all be helpful for understanding, and it doesn't really matter that the market isn't of one mind, or that some weak hands have joined in the same direction with the strong hands and all that.
, How far do you go back with that? The "bloke who was short on the other side of your long" may, too, just have been unloading a position mayn't he?
