What I meant was - most emails don't include deep links. Take LinkedIn for example - if you click a CTA from a LinkedIn email, it takes you into a browser (even if you have the app on your phone).
Most emails still function this way. It doesn't work because while the user might be logged into your iPhone app, they're likely not logged into your website in mobile safari/chrome.
Emails are a good way to notify you, if you want to still go search/navigate. They don't take you right to checkout though, which is negative for conversion.