This can never be true in theory or the real world . if buyer A bought some coins at $1 and sold them to B for $2 then A has made a profit of $1 . when buyer B now decides to sell and based on market conditions sells for $5 dollars then he would have made a profit of $3, are you suggesting that buyer B made a profit at the expenses of buyer A, this can absolutely not be the case so to say "profit for one investor means loss to the other one" is in fact a statement that is not factual. both individuals come come out making profits.
There's just one flaw in your example: Who bought from Buyer B? Yes, Buyer A and Buyer B made profits, but in order for that to happen, there has to be a Buyer C (or D, E, F, etc.). Not all sellers will profit, as we all know that somebody that FOMO'd at $20k and then panic-sold at $10k, but that's just the nature of the game. And yes, this
can absolutely be true in theory AND the real world. A prime example of this happening on a day-to-day basis? The stock market.