https://wccftech.com/wear-os-3-will-work-with-iphones-despite-galaxy-watch-4-not-supporting-ios/

The Galaxy Watch 4 was the first smartwatch to ship with the latest Wear OS 3 and therefore, paved a way for more Wear OS 3 watches to come out in the future and actually allow Google to have a fighting chance against Apple's growing market dominance in terms of wearable. However, the most surprising thing was that Galaxy Watch 4 did not have support for iOS. Which means that you'd need an Android phone to use smartwatch.

Now, in a surprising turn of events, we now have confirmation that Wear OS 3 will have support for iOS, and users will be able to use it without any issues.