The page says that it's "Powered by Google's Filament renderer" which is written in C++, and that Fluorite itself has "a data-oriented ECS (Entity-Component-System) architecture ... written in C++".

Also, although Flutter/Dart applications can run on web (compile to WASM or transpile to Javascript) and so can Filament, the Fluorite FOSDEM page says target platforms are "mobile, desktop, embedded and console platforms" so it's not clear Fluorite even cares about running in browser.