I think author put incorrect words into the statement you are quoting. Flutter has much less mature environment, UI packages look good, but most are hot garbage without even unit tests. You can't trust even a single one if it doesn't have at least 5 active developers. They are drying like flies, no one maintains them, updates on time and Dart package manager doesn't help at all, makes things worse by having that system of requirements, versions. I used to work on a side project in Dart and Flutter and quickly fall in love, because i was able to write UI and backend code super quickly, but about 18 months later, I was spending more time maintaining packages that were dropped by original authors, updating, fixing conflicts between versions, and this was the reason I simply rewrote everything back to Java, even frontend to PrimeFaces (: Nothing broke by itself since early 2021.
Even Google dropped 3 packages I used in my projects. Including Dart Angular, they archived it 7 months after promising in AMA they will maintain it for a decade ahead (: They also abandoned their flutter charts library, it stopped working in newer versions of Dart and community had to fork it, they made like 80 forks in a week.
I think this is the problem author is trying to say, not Flutter as the framework, but as the general ecosystem that's consistent and provides at least somewhat positive experience using it. Google is neglecting it.