I'm not sure that is the case, manifestos are usually more tactical than strategic.
I think Labour had 2017 on their mind: apart from the policy and communications carnage of May's campaign, 'For the Many Not the Few' cut through surprisingly well to the electorate (this certainly surprised Labour more than they now like to admit). So when I say they bet the house on the manifesto, I mean in the sense that they actually believed they could repeat the strategy, and that doing it bigger would translate into a grand endorsement, a majority, maybe a dream landslide. It could be 1945 again: 'It's Time For Real Change' was designed to be their 'Let Us Face the Future'. The difference is, Labour in 1945 wrote their manifesto with zero conviction that they would actually beat Churchill straight after the war, and so let loose; whereas Labour 2019 have been organising their campaign with an unreal, indomitable optimism and will-to-believe. The first strategy was to circumvent Brexit (and Jews) by making the electorate an offer they could not refuse, and there is no reason why this would or could not work. But it doesn't look like it has, and their switch towards negative campaigning on the NHS is a tacit admission that was the case: they deployed Plan B. All I am saying is that if they lose then their time will be better spent trying to work out why the manifesto didn't work, rather than creating scapegoats and purging each other ("melts" etc.).