kubota will not warranty parts that are not purchased from a kubota dealer. I'm not sure of any manufacturer that will, so when one makes that kind of statement-it certainly makes one question their credibility.
BTW unless the original injectors have had water or other contamination run through em, they're good for a 5 year emissions warranty through kubota.
also you have to program the new injector's data into the ecu in order for everything to work as designed.