What the conspiracy theory never explains is why Corbyn did better than expected when “Blairite" Labour officials helped run the 2017 campaign, but went down to ignominious defeat in 2019 when Corbyn was in total control. https://www.theguardian.com/politics/2020/aug/07/jeremy-corbyn-accuses-labour-officials-of-sabotaging-election-campaign

Corbyn is constructing a classic stab-in-the-back myth for his supporters "You haven’t wasted your time and energy. Stick to the cause and we can try for domination again. Only the machinations of traitors stopped us winning in 2017. Next time there will be no mistake."

I wrote about it here. As an account of what happened, it makes no sense. But as a method of keeping supporters in line and loyal to the Corbyn cause it's close to brilliant https://thecritic.co.uk/issues/june-2020/the-stab-in-the-back-myth/