It's not available in the apps, according to the community post announcing it. I have already made a submission to Report a Problem to find out why. I left Khan Academy in late 2021 after almost 4 1/2 years of barely tolerating this inability to restart tests, quizzes, etc. It finally became too frustrating to me. I was ready to plan a return to Khan for later this year until I read the community announcement. But unless a rollout to the apps is imminent, I won't be back after all.
(I learned about this new feature because I had notifications turned on for email notifications of posts to which I had contributed to over the years.)