Weird statement by the company who created flutter.
The thinking was that having portable widgets was not a good thing and for the highest quality app you still want native UI, while having portable "model" [ as in model in model view controller ] code.
Flutter was developed by a different team, and has a different philosophy about this - and in part this is driven by solving for a different goal. For super sophisticated complex apps with large teams, native is still the way to go. If you look at all of Google's own major apps, they are all native.
The majority of their revenue by way of ads is handled via AngularDart in the web interface and Flutter on mobile devices.
They literally put billions of dollars of mission critical stuff into Flutter at this point. Same with the entire Google Pay ecosystem and soon to be with Fuschia.
I don’t think Flutter is a great tool for a bunch of use cases but that list is shrinking a lot with each release. It certainly has all the moving parts needed in terms of technical capabilities to investment to make it a default choice for a lot of things in the future.
You still have to staff up your platform teams, though, and now you have the cross-language barrier to deal with in each code base, so each team needs expertise in that as well.
Flutter lets you have just one team write one app, maybe with different themes per platform, but it's still all in the same language and runtime. The cost being that you get not-quite-native UI and performance, but maybe that's an OK trade-off.