"Flutter is optimized for 2D mobile apps that want to run in both Android and iOS. Apps that use Material Design are particularly well suited for Flutter."
Which means that, if Flutter succeeds, iOS users should expect to see more and more apps built using Google's Material Design language. It's already happening for some of Google's iOS apps--the floating action button in Calendar, Hangouts, Photos, Docs, the iconset in most of their iOS apps (Gmail and Chrome are partial exceptions), etc.
In other words, Material Design is making significant inroads into iOS. But Apple's design language isn't doing the same in Android world.
Disclaimer: I'm a cofounder of Recent News (https://recent.io/), and we borrowed some Material Design concepts for the iOS version of our app.
You may see that for big must have apps from Google like gmail or Maps where the user has little choice. But if indie developers try pushing Android-y looking apps on iOS I would suspect most of them would lose out to an app that utilizes the native design language.
I don't how long ago you started using Android, but iOS design language was in a lot of Android apps. For example, a "Back" button on the top left corner when Android had a hardware (later software) back button, and navigation controls where normally at the bottom. I blame designer's who insisted on 'direct' ports of iPhone apps.
Too many designers worship on the alter of Apple: I remember when brushed aluminium backgrounds where the rage (even on websites), this was later replaced by "linen" backgrounds and gossamer-thin font-weights everywhere. So I beg to differ: Apple's design language was making "significant inroads" on many platforms (web, native windows apps), not just Android.
I must confess I feel a bit of schadenfreude on the turned tables.
They have a number of Material Design web components that are very easy to integrate into your site but also more difficult for people to override.