It's a bit of a mish mash in terms of what they're saying, and how the examples are being mixed up with each other
Which bit are you disagreeing with?
Historically speaking, the international jihadis do have a lot in common with the International Brigades of Spain, but only in terms of the way they are recruited and organised.
Both rely/ relied on civilian volunteers to come forward and travel to the theatre to take up arms and fight for a cause. Both involved motivational speakers (platform, or more recently of course, cyberspace). Both then relied on a network of agents/ fixers arranging passage for them. Benn would be right to say that it was mainly the working classes and the trade unions that supplied the International Brigadists, but others were drawn from middle class professions. There are echoes here too with the social class of those travelling to join ISIL. Benn also omitted to say that many of the international brigadists were arrested on their return to the UK (more so in the US where the Abraham Lincoln brigade faced all sorts of punishment).
Where the distinction occurs of course is the enemy. Those fighting in Spain did so in support of a democratically elected government in the face of fascism, albeit the opposition groups had completely splintered and ended up fighting each other, again an eerie echo of what Syria looks like today. I've drawn the paralell with Spain myself on a few occasions, (and long before Benn mentioned it).
However, there is a distinction. Those who've become international jihadists can't exactly claim that they're fighting in support of democracy, and it is here where I would agree with Benn that they're answering what is essentially a fascist calling, but it would still fall within the broad confines of an international.
I think a lot of the principles are similar, albeit the composition of the groups and the causes they're fighting in support of, are polar opposites.
I should perhaps also point out that Franco also succeeded in generating his own volunteers from European countries, mainly those who fought in support of the Catholic church. If Benn was looking for a better comparison than it would be something like the Irish Brigade of Eion O'Duffy, who were moved by a religious calling, and who fought on the side of fascism. Invoking these groups however, wouldn't fit the parliamentary narrative he was hoping to appeal to