I wondered that @user8800 but I think the rationale will be he's changed the Labour Party including booting Corbyn out. Ergo they are now launching a fully costed manifesto.
The Tories can say 'oh look you contributed to that bad manifesto' but then the Labour line is 'yup but we've changed' which is something they want to keep front and centre. And they can add 'you agree your manifesto is full of last minute uncosted pledges then?' which is going to be their main attack line.
If the Tories are stupid enough to fall into this bear trap it means the main take aways for anyone not interested in politics are 'Labour has changed' and 'Tory manifesto uncosted' and they don't get any publicity for whatever is in there.
If they have any sense they say nothing and keep shouting about whatever is in there they want to shout about like reducing NI and protecting pensions. But they don't have any sense so expect 24hrs of 'but Jeremy Corbyn...'