I am not surprised you haven't heard of flutter until now; its a relatively new technology and the dart programming language was burred after its release for various reasons, mainly because people didn't like the idea of it replacing JavaScript (which actually wasn't the original goal).
Well, there are various reasons why some one would choose flutter, the main draw being that you make a mobile app that uses one single code base (for both iOS and Android). The performance of Flutter is also very good and on par with native solutions. I personally like it because of the abstractions and the general method of doing things; decoupling the data layer from the UI and logic and using immutable widgets for everything etc.
Utopian has a ton of people (it is based on the open source community) and the SteemStem group is fairly nice as well; they've got some really intelligent people who write for them (scientists and mathematicians).
Anyhow, glad you enjoyed the content.