Oh my goodness. I tried opening that sample app on mobile and it was like interacting with a touchscreen ticket machine from 2005. I don't know if it was just broken on my browser (firefox android), or expects me to have the latest flagship to actually use it.
Regarding the semantic HTML stuff, I don't think that's an issue inherent to the design, since flutter widgets tend to be highly semantic in nature. In fact, I think a platform like this could good for accessibility if it handles it properly instead of leaving it up to fallible developers.