Oh well, if Alan Johnson (top Blairite) said all that, who can possibly argue.
Corbyn and Momentum were two of the reasons that Labour in 2017 got 40% of the vote and it’s biggest vote share rise since 1945 - and with a very popular manifesto. Not a success - Labour didn’t win - but they came close so certainly not a disaster.
2019, with the same leader and similar manifesto they got 32% and lost badly. Corbyn was certainly less popular - but that was mainly due to 2 more years of personal attacks from the press and Labour right (including Johnson) and an incoherent policy on Brexit (forced on him by the Labour right including Starmer and Johnson)!