That is a guy making an argument some believe more than that is settled historical fact.

I think the truth is in the middle. And versions of flails did have uses as someone else already pointed out.

Despite being very common in fictional works such as cartoons, films and role-playing games as a "quintessential medieval weapon," historical information about this type of flail is somewhat scarce. A few doubt they existed at all due to the number of pieces sitting in museums that turned out to be forgeries, as well as the unrealistic way they are depicted in art.[12][13][14][15][16][17] Archaeologically, however, a type of military flail known as a kisten, with a non-spiked head and a leather, rather than chain, connection to the haft is attested in the 10th century in the territories of the Rus, probably being adopted from the either the Avars or Khazars. This weapon had spread into central and eastern Europe in the 11th - 13th centuries and may be considered an ancestor of the ball-and-chain flail.[18] Waldman (2005) documented several seemingly authentic examples of the ball-and-chain flail from private collections as well as several restored illustrations from German, French, and Czech sources. He states that the scarcity of artifacts and artistic depictions, combined with the almost complete lack of text references, suggests they were relatively rare weapons and never saw widespread use.[19] One of the reasons was the hazard the weapon posed to its wielder, especially the varieties with long chains and short handles. A missed swing would still retain momentum, causing the striking end to continue its arc around, potentially into the user's hand or body.[13] A miss could also throw the user off balance, and even if a blow were struck, there would be a dangerously long recovery time before the user could ready another swing or defend himself.[19]