The short response is, of course, "it depends."
There are essentially only two types of economic activity: creation of wealth and trade. Creation of wealth is actual manufacturing or agriculture that generates consumables or what I would call "productive infrastructure" like machines or homes or storage facilities. Trade - while it may indeed generate value for both parties involved in the trade, cannot generate wealth.
In the general economy is a combination of activities where some people / organizations do actually invest in new production to create wealth so in aggregate it is true that the economy is not "zero sum". But at some point things bifurcate: a portion of the population is no longer able (for some reason or another) to generate new wealth, but instead can only trade.
For the portion of the population that can only trade, it is indeed a zero-sum game, except for the instances where the actual wealth producers inject new wealth into the trade markets.
So yeah, a Gates or Buffet or whatever doesn't necessarily prevent new wealth from being created - but as wealth concentrates, there isn't necessarily any impetus for the wealthy to generate more wealth (in fact, diminishing marginal returns say at some point it won't). We are just assuming that the wealthy will continue to want more - but at some point, they are effectively self-contained and would no longer have any need for trade. At that point - what happens? This is the source of fear (rational or not) from globalization and automation - when the owners of productive capital no longer have a need or desire to trade with non-owners.
To summarize - and I do agree it is theoretical/hypothetical: super-wealthy entities harm non-wealthy when, instead of using that wealth to increase production (through productivity increases or just new capacity at the same productivity levels), that wealth is only used to acquire ownership of a larger portion of the total available wealth (e.g., rent-seeking).