Isn't a big part of the point about the ISG's analysis that trade unionism/the labour movement is basically finished ("no second coming of industrial unionism") that it's an attempt to theorise a thoroughgoing retreat from class?
As I understand it, the ISG now sees nebulous "movements" (e.g. Occupy, Indignados, etc.) as the key agents for social change, rather than organised labour.
It's not just about a misunderstanding of the historical role of syndicalism or whatever (this is a sideshow here; frankly, syndicalism would be an enormous advance on these people's politics), it's about reaching an analysis which actually breaks from Marxism entirely. If you think that labour is no longer the key, primary, privileged, fundamental (pick whichever word you want; I think they all apply) agent of change because of its unique role in capitalism, then you have to conclude that Marx was actually wrong about the labour theory of value, surplus, the wage relation, and pretty much everything else.
It's not just the odd departure from "the Marxist tradition" here and there; they're moving in the diametrically opposite direction.