Whether it is true or not that in markets like Bitcoin (crypto) ; one person gains only when the other losses some thing. Suppose one person bought one Bitcoin for $5000 in January 2017 and sells in June 2017 for suppose $10000 he gains $5000 by selling. Now the purchaser in June 2017 @ $10000 sells it for $6000 in January 2018 and as a result he losses $4000. So it is clear from the example that the profit of seller in June which was $5000 includes the loss suffered by the person who bought it in June 2017 and sold in January 2018.
If there are some other reasons for this profit / loss factor, I request other members to please share their knowledge with the platform because my opinion may be on the basis of partial truth and there may be other factors responsible too.
You are perfectly right that transacting is a game with negative expected return. Why is that? Because the only sure winner is the exchange or broker who takes profit from percentage or constant fees. Bitcoin is still different, but if you take forex, than the next winners are big corporate investors who have their infrastructure enabling them a very fast access to that market, they can make arbitrage or take positions increasing liquidity which give them profit because they are the very first to do it. The rest of investors don't have those opportunities, they are to slow, and in the long run they lose (negative expected return).
In bitcoin the expected return is still negative, but with current influx of money into bitcoin you can bet against the increase of the price (aka hodl). It may be a good bet, I admit. But chances are, of course, that you will lose. In that case the winners will be the guys who sold when it was still high and maybe will buy on the next deep.
Remember, that the expected return is negative, what means that it is not "when you win somebody must lose", but rather "somebody must lose some more when you win", and of course, "when somebody wins you lose some more".