He mentions George Orwell, probably the most famous British socialist who fought in Spain, and wrongly ties him with the International Brigade. George Orwell entered Spain by using contacts he had with the Independent Labour Party (ILP), (a separate political party to the Labour Party). He joined the militia of the ILP's fraternal party in Spain, POUM as part of the `ILP Contingent', and fought on the front lines with them.  

The International Brigade was set up and controlled by the communist party, specifically the Russian communist party. It attracted volunteers from around the world, including as Rosen says, Jack Jones, ex-General Secretary of the TGWU. While many units of the Brigade did fight the Fascists bravely, and lost many men doing so, there was a darker side to its activities. Some units of the Brigade were used to kidnap torture and kill militants of political groups that the Russian communists considered rivals.

George Orwell and the `ILP Contingent' took part in heavy fighting in May 1937 when the militias of POUM and the anarchist's were attacked by forces under the control of the communist party. Following the fighting the POUM was suppressed, many of its leaders being arrested tortured and killed. Following Orwell's escape from Spain he found out he was targeted for death.  

During the civil war and after the communist parties denied that they had been engaged in torturing and killing members of political parties ostensibly on the same side. But there is a wealth of documentation to prove otherwise, vividly in Orwell's Homage to Catalonia, and more recently in Ken Loach's film Land and Freedom.
Does all this matter now?

Well if the truth doesn't matter to a history group who does it matter to?