I think you’re misreading this one. I don’t think they’re saying “opening a browser sets it to your default”, they’re saying that when the OS prompts you to select a default / when the user changes the default, it should automatically download that browser application for the user. This in contrast to either A) only showing you which browsers are already installed or B) showing you options, taking you to the App Store for the replacement if it’s not already installed and then requiring you to return to the selection to select the new browser after it’s installed.