Aside from that article being quite old, the newest version of Flutter has tons of fixes for iOS. Although at the moment it's mostly used for internal apps.
Facebook also realized that they had to go full native a long time ago.
You should always worry when your platform vendor doesn’t eat their own dog food. Besides Flutter will always be behind tsking advantage of the latest iOS features - especially if Google doesn’t depend on it.
This is also one of the major complaints about some of Apple’s APIs especially on the Watch. Even worse, Apple has specialized equipment that makes Watch development faster than it is on the outside.
(Disclosure: I work on the Flutter team.)