Apple has an advantage in numbers reporting. They sell a limited number of variations so when looking at sales of a single device, the Apple Watch is the best-selling single device of its class (smartwatch). However, there are possibly more Android Wear watches sold as a group.
The same is said for iPhones. While the iPhone sells many more than any single Android device, there are a many more Android devices sold as a whole.
So, yes, Apple can tout their devices as the #1 selling device when you compare them to any other single device rather than to a competitor's ecosystem as a whole