If its true, then why does what we can observe empirically about trade (the erosion of the middle class job base that directly corresponds with the rise of that same job base in China, and directly corresponds with the radical rise of Chinese wealth and the Chinese economy) indicate it isn't true, whereas the only thing that indicates your beliefs are true are 'theories' and 'experts'?
Here, I'll even save you the time.
Ask the Harvard MBA: Is global econonmics a zero sum game? OF COURSE NOT, he says!
This guy at Columbia Business School likewise agrees that trade is not zero sum!
As an aside, after the re-crash in 09, Nassem Taleb talked at length about people who persist with theoretical mantras in the face of empirical evidence that indicates their theories have problems. The video seems to have been deleted from that page but I'll go find it. Its a worthwhile watch.
If you demand a pure and absolute definition of zero sum, I suppose the idea that Zero Sum trade isn't true can be compellingly rationalized (since there are instances where it's mutually beneficial), but between the idea that trade is absolute zero sum versus the opposite end of the spectrum, where you are, that free trade is mutually beneficial' by some kind of default, exists a nuanced position (my position) that holds trade policy can be strategized, managed and that free trade suicide pacts are just as damaging and economically detrimental as reflexive and mindless trade protectionism... and on the matter of managing national interests, getting the better
end of a trade deal is just fine, even if it comes at the direct expense of our trading partner. You seem willfully naive to the idea that there is even such a thing as a 'better end' to a trade deal, since you're operating on the ideological mantra that "free trade is mutually beneficial", actual facts be damned.