Apparently the rewrite of this dry wall of text is not live yet, so let me paraphrase:
You contact us and we book you the flight that gets you to your destination the earliest (if needed we pay for a taxi or bus or anything to get you to another airport if needed.)
This doesn't sound like a 100% guarantee. Most of the times on the day tickets will be a lot more expensive than advance purchases.
I'm not sure about the details since I don't have much to do with the legal side of things, but I would assume that Kiwi.com had to put this clause in to avoid having to pay when a flight's price is 1000x higher than it should be as an error fare, or similar cases. I've personally never heard of a case where expenses weren't fully covered because of this clause.