I am trying to say that the Contractor is giving the warranty on his/her work using Hunter/Rainbird parts. Design and installation are very important for low maintenance sprinkler systems.
I think you are getting hung up on RainBird or Hunter. The real warranty that you need to focus on is from the contractor because Rainbird will not come out to your house and replace a sprinkler head that is incorrectly installed or defective.
The contractor that you choose to hire will or will not. They all say that they will, but it is not usually the case that all contractors earn a good reputation from providing warranty work.
Thanks
I hope this was helpful.