The article is missing the point of flutter. It's designed to build apps, not semantic web. No way flutter apps will be indexable. I believe one of the web rendering backends(there are 2) is using webgl and compiles to webassembly, so there goes your divs...
But I must say as an app programming platform, flutter is brilliant. The ability to pass around statically typed classes representing everything from widget to text style to animation is so fast and logical, that it actually made me enjoy frontend programming. No need to mess around with html+js+css+some macro language to glue it together.
Did I mention i run the same flutter app on android, ios, web and linux box? I just wish they would use something like kotlin instead of dart, but still, it's well worth it.