Simple systems acting in concert are devilishly complex. (Former algorithmic trader.)
Counterparties' systems fail. They misfire. They get confused by the weather and interns punching in parameters wrong. Internal dynamics, like someone stretching to hit a goal or getting lucky on a hedging contract, are material and unpredictable.
And that's the least of it. The limits on the sorts of adversarial behaviors that manifest when counterparties know you have an algorithm watching are profound. It incentivises betting against the watchman. Google may have the data. But the airlines control the book.
But I do not believe so. From my experience airlines are extremely strict about their partners displaying correct pricing. I've been told displaying a price lower than what is available is illegal / a liability and work under that assumption but I'm not the person who knows the specifics. However displaying a price higher than the lowest available will also get us in big trouble with the airlines. Maybe my work is biased since I deal with promotional campaigns and ads, but the general reason for a complaint will be something like (just making up fake examples) the airline having a 24-hour sale, and because we our systems didn't pick up on that for 2 hours so that's 1/12 their marketing budget thrown away.
Furthermore, many airlines don't directly manage their own booking and pricing and instead have it managed by a third party. So aside from legality and conditions from the airline, you can also be bound to conditions by your reservation system. There are plenty of them, but if you want one to Google for your curiosity Sabre Corporation is the largest.
Google does have a tremendous advantage if it controls the _flow_( sending traffic/customers to airlines) versus the airline's having control of the book.
It's not a simple problem. It's unlikely Google has it remotely solved.
> Google does have a tremendous advantage if it controls the _flow_( sending traffic/customers to airlines) versus the airline's having control of the book
Google is one of many online travel agents for airlines. It doesn't control the flow.
"Controlling the book" means airlines know who has booked how many tickets and at what price. They control how many tickets are left. They control whether and when a flight flies. They control prices.
Reselling tickets is a distribution problem. Guaranteeing prices is a trading problem. Google has a pitch with the former. It holds few cards with the latter. The only stick it holds to dissuade aggressive behavior from airlines is cutting them off from its distribution.
In June, I was literally in CVG airport (inside of security) when my Delta flight was delayed. I decided not to book a ticket on United that was leaving in ~35 minutes because I wasn't sure that I could get over to the other terminal before boarding closed. (It also wasn't 100% clear that the United flight was going to make the connection in DC because of weather that was also screwing up Delta.)
If they only guarantee the price of the same airline (which make sense considering that different airline offer different quality of service), then that would just hurt Google once without benefiting the airline and in fact, it would hurt the airline in Google.
The terms are clear: When we predict the price won’t decrease for select itinerarie
If airlines have unpredictable random price spikes, then they won't be able to qualify for the price guarantee. Having that guarantee seems like a good value add and may bring more customer toward theses offers. I don't know if Google would place theses airline higher in the result, but it would make sense because that's something that someone may prefer.
It was part of their usual shady promotions where they say "flights from $199" and there are really only a couple of seats available at that price, and they go fast.
The 'best price guarantee' guarantees the customer will receive the lowest price for their ticket. Google will refund them if anyone gets a lower price.
The airline can release, for 30 seconds only, a much lower price. Then Google has to refund all their customers, and the airline might only sell a handful of tickets at the lower price.
Remember airline tickets aren't fungible. They have names attached, and can't be resold, so nobody can buy up thousands of tickets in a price dip and resell.
"Prices are measured between the purchase date and the departure date of your eligible itinerary." 
The key things to interpret there are "between" and "departure date". For example, I would interpret that as price changes on the day of the flight don't count. (i.e between is exclusive and date means the date, not date and time).
The airlines really are shooting themselves in the foot though... Google will end up with the biggest slice of the pie...