They haven't offered the impossible - they've not offered anything at all. The idea that the uncertainty of Brexit offers a valid excuse for doing so is a load of shite. Every party can face uncertainty after an election; they're still expected to come up with costed plans as to how they intend to manage the economy. The Tories haven't bothered their arses to do so.
For what it's worth I think there's a lot that can be criticised concerning Corbyn's manifesto: it's perhaps too ambitious and a free-market leaning voter would argue it may drive business away from Britain during the Brexit process, but the whole Labour argument is centred around the idea that austerity and cuts haven't worked for the general working population as a whole, and that something substantial is needed to reverse this. That's fair enough. You may disagree with it, but at least they've come up with a costed, albeit ambitious and perhaps overly ambitious plan for what they want to do.
The Tories haven't done any of that. Rudd quite literally defended it by saying people should look at her parties record...which is...well, what the feck does that mean? They've pretty much neglected policy in this election because they presumed they'd win, and because they're trying to argue we have to have May negotiating Brexit because...well, I'm not sure, really.