I typed in “python flutter” into Google and clicked on the first link. The first pictures shows a bit of code and a simple window with two buttons. I go back to the code and skim it. It defines the buttons. How you cannot deduce from that, that this library makes UI says a lot about you.
I also think your assumption that click replaced argparse is wrong. Click heavily relies on decorators which makes separation of functional code and command line interface code either impossible or difficult. If you only care about your one program that is fine, but it does make your code not very reusable.