You're misquoting.According to McDonnell their current leadership see "a transformative, some would say revolutionary party, aiming at the radical replacement of the existing economic and social system." If they gain power they will implement a programme of spending and asset appropriation on an unprecedented scale which will quickly destroy our wealth creation base.
The question posed was "whether the Labour Party is a party of social reform aiming simply to ameliorate our existing capitalist society, or a reformist party that seeks to replace capitalism by incremental social reform, or a transformative, some would say revolutionary party, aiming at the radical replacement of the existing economic and social system".
His conclusion? "The history of Labour holds open the possibility that the party could move beyond social reform and become a genuinely transformative party once again. A party leadership under Jeremy Corbyn, and a mass membership shaped by the experience of the economic crash, the years of grinding austerity, and its resultant inequality and injustices, are taking their place in the history of the party." That's rather more circumspect. Anyway, if people voted for "transformative" policies, what's the problem? Do things not need transforming?