Maybe they put the warnings down to "right wing biased sources" and didn't believe non-doms would leave.
Or they knew the warnings were accurate but the policy would help them get elected and it'd be fine, they'd think of something to raise money in other ways once in power.
The second possibility is risky - they're in danger of having to raise / save money in ways that piss off people who can't just leave, unlike non-doms, but who do have votes.
I mean, you can see their logic with VAT on private schools and axing the Winter Fuel Allowance for many pensioners - most of them vote Tory, they thought, so Labour won't lose votes it already had. Though events at the Labour conference should have given them food for thought
So what do they do? Take cutting / tax-raising measures that affect (even more) possible Labour voters? Keep blaming the Tories for five years? Hope the economy perks up in five years time and everyone feels better off (and that was the entire basis of their manifesto)?