Upvote:2
If you make a booking with an airline, and they then significantly change the flight details (typically changing departure or arrival by more than a few hours) then the normal minimum standard is that they must give you a full refund. This is based on established practice and basic legality - if I pay you money for something, and you can't deliver that something, then you have to give me the money back.
Airlines are reluctant to talk about this full refund, and will do a lot to persuade you to take another flight with them - after all, they want your money. But it is virtually incontrovertible that they have to give you a full refund. They are unfortunately not obliged to do more than that, even though finding an alternative flight might have become more expensive by the time the change/cancellation occurs.
In the case of Kiwi, Kiwi do say very clearly that they will give you an alternative or a refund. They don't say in the guarantee that it will be an acceptable flight, but I think a lawyer is going to have a field day if they only offer you a completely unacceptable alternative and refuse to give you a refund.