What this means is that Stadia will be playable on iOS devices through Safari, as opposed to a dedicated iOS application. This is kind of a loophole which will allow Google to bring Stadia officially onto iOS devices, but at the same time allows them to skirt around some of the rules that Apple requires for game streaming services.
Apple has relaxed their stance somewhat regarding game streaming services, but they also introduced new rules that developers need to meet in order to have their game streaming app approved for the App Store. These rules, by and large, are still extremely restrictive and we’re not sure if they’re entirely realistic as it would require the games available on the service to also be available as a standalone app on the App Store (for review purposes).
This is why Google’s workaround seems to be a decent compromise, although we’re not sure if streaming Stadia through a dedicated app would provide a better experience compared to Safari. Google has not stated when this web app will be released, except that it will be in the coming weeks.