Yes, once your exam is on the phone: you can practise and sit mock exams without a connection.
The app keeps your exam's content and your answers on the phone. Offline, it shows the content it last checked. What you answer is kept on the phone and sent to your account when you are back online; while the app is open, it tries again by itself every 30 seconds or so.
Some things need a connection: the first start, before any content is on the phone; the check; signing in and out; buying and restoring purchases; setting your exam date; seeing your other exams; and deleting your account.
You can only sign out once every answer has been sent, so nothing you did offline is lost that way.