Sure, but they are correct and you're focused on the premature optimization.
I've also built a small flutter app where everything runs snappy and it more than solved the needs with a minimal development time and number of lines of code.
If you’re happy and your users are happy with Flutter, go right ahead with it. But please don’t be the all-too-common startup that writes blog posts about their “coding velocity” with Flutter (of course, they can’t afford to hire talent for anything else) while they keep their fingers in their ears as users scream about usability or missing platform functionality.
Then you’re probably fine :) Do make sure to keep your eye on this, though, just like you might be a bit more regular when checking the performance of a Python application.
I've also built a small flutter app where everything runs snappy and it more than solved the needs with a minimal development time and number of lines of code.