a product experience is sum of all things !
you encountered one part of Ola, wherein the driver canceled the booking and then in other part, you experienced the Ola, wherein the other driver saved you from missing the flight.
These are two dimensions of Ola or the product for that matter.
May be you can average out everything, by awarding one star for the canceled ride and five star for the airport one.
One of them you liked, the other one you hated.
But I believe, that would not be correct.
On a more technical aspect, your rating was asked only for the ride which you completed and not for the canceled one.
So, you have to rate, the ride and driver, which got you to the airport. In that case, I presume that it would be five.
On the second aspect of rating of SLA, it already happens, when you complete a ride. So when OLA asks you to rate the driver, essentially it is the entire experience of ride, including SLA (if you award low ratings, it asks you for the reasons, and you can gauge from it, that is indeed about the entire experience and not only the driver).
SLA gets enforced only when you are served. How will it gets enforced when, you are not served?
In your case, when the first driver canceled the ride, you never availed of the service. So the question of rating it, does not comes.
Probably this would be done to prevent mischievous claims and frivolous lawsuits.
I have not read the T&C, but perhaps this explains it.