On iOS, you tap the profile, then tap “more”, then tap “start secret chat”. So depending on whether navigating to the profile counts, it’s also three taps. I agree it could be more direct, and should be included in the “New message” menu accessible from the pencil-on-paper icon in the top right of the main screen. I’ve used telegram on many platforms and the UI is in fact quite different. The iOS and android apps are totally different with different features, as are the Mac app, cross platform desktop app, and various web apps.