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.