Once you’re on an Android with virtual bottom buttons, nigh-most apps – the Outlook app, for example – will go full screen at some point and you have to poke them to get the bottom buttons back. The stuff around the front camera notch doesn’t go away with those (though it does with sky maps like Stellarium).
I think “full screen” is being poorly defined here; the notion of the screen is increasingly virtual in recent Android, to the point where you need to run the moral equivalent of a window manager to get the bottom row buttons at all.
My own take on it is that Flowx is not doing anything surprising or unusual, especially for a data visualization app. I think much of the problem may be that people are looking at Flowx as a weather app, which, fundamentally, it is not. It’s a data visualization app that happens to be about weather. That’s what makes it interesting and useful, and it’s probably something to tell people up front. (So is “this is a powerful tool, and the power comes with a certain minimum complexity”. I think the main selling point is that the complexity is brilliantly handled, but it’s still there.)