Very rarely do the people working full time on a framework/tool also use that framework/tool in a non-toy setting. They aren't working two full time jobs after all.
Are you consider a dogfooder if you use the browser? or do you need to lots of write Javascript yourself, etc. to be considered "a user of your product"?
Typically, these are two different sets of people.
So, I don't buy the "always, always" part
Nor are the people doing UI work in Blender going to be able to make Big Bunny.
Nor are the people working on fusion360's parametric system going to build complex mechanical-fluid simulation scenarios.
Flutter is simpler than those, sure, but even in that world you have people that do nothing but fonts & text for their entire careers. They'll know how to do text stuff in flutter, but they'd be lost if you demanded they make a tiktok clone.