it looks like these APIs are a fairly minimal extension of existing functionality for third-party WebKit-based browsers to also cover other engines. this is clearly the usecase they're intended for though
> To add a bookmark to someone’s Home Screen from your browser app, create an object that conforms to SFAddToHomeScreenActivityItem and present a UIActivityViewController that includes the activity item.
> [...]
> If your browser app includes an alternative browser engine, pass detailed information about the bookmark to getHomeScreenWebAppInfo(completionHandler:). If the bookmark represents a web app, include the web app manifest, and cookies that the system uses when someone opens the web app from their Home Screen.