I think he meant that 'New Labour' were rendered unelectable. That's not really the case. In 2010 they had a manager rather than a leader in the form of Gordon Brown and the country was in a recession for which they would inevitably take some of the blame. Despite that the Tories needed to form a coalition government.
The more interesting issue is why Labour didn't win in 2015 and 2017, after years of austerity and lack lustre economic performance and with the Tories seemingly choosing not to contest the 2017 competition.. I'd argue that they didn't have leaders who inspire enough people in the country, just as in 2010.