The 'double coincidence of wants' problem of barter exists. What is a myth is the assumption that any culture used barter as a primary means of exchange. Barter was used on the edges of tribes, clans, etc. between groups of people who do not share the kind of trust that exists between members of groups, who will likely live out the entirety of their lives around each other.
That does not take away from the fact that the double coincidence of wants is not a myth. A lot of card trades depended on both party having a card that the other party wanted when no monetary medium was used. I am not saying that this was always the case but it happened often.
The realization that there is (a) a double coincidence of wants and (b) this can be solved with some sort of currency are absolutely questions already answered if not intuitively than explicitly by money in modern society, as are related concepts like debt and interest. When someone talks about the double coincidence of wants being a myth, they are not referring to the obvious fact that this is a problem money solves, but that this reason, alone or primarilty, is the reason money, as a world-historical concept, came to be in all societies.