It's zero sum the way a poker table in Vegas is zero sum. Every dollar won is a dollar lost by someone else. Meanwhile, the casino takes their rake.
Eh - the poker table doesn't have a concept of dividends. (A free drink or two not withstanding)
This isn’t always true though. If I buy an equity for 10$ and sell it to you for 11$ who then sells it for 13$ none of us have lost money. Only panic sells and crashes are zero sum in the way you are imagining.