Manky does the app send telemetry upon opening
The app can send telemetry at any time it's running. The reason Mysk's findings about push notifications is news is because, typically, apps in the background on iOS are not actually running. However, they can briefly run when receiving a push notification. This is supposed to be to enable enriching the notification, such as downloading or selecting an image/icon to display, or to decrypt the notification payload if it's encrypted. But instead, data hungry apps are using that brief time iOS gives it to send even more telemetry.
Can an app send telemetry? Yes, at any time it's running, especially if it's in the foreground. Does it? That depends on the app and is simply not something you can know without MITM the traffic.