Any company building their business on flutter is setting themselves up for disaster and an expensive double-rewrite to native, while having a useless engineering team because they skilled up previously mentioned dead-end.
I’m relatively confident that they used a device frame around their actual screenshots, which leads back to those issues you mentioned. You can tell that they’ve used different device frames in their current store listings compared to the realistic looking ones on the aforementioned Flutter showcase page.
This whole process can even be automated. I’m talking about a CI/CD workflow that takes screenshots of specific pages, and adding device frames around them etc. See this tool for example: https://docs.fastlane.tools/actions/frameit/