I don’t really understand Canonical’s decisions. Early on Ubuntu really did make things simpler, but now days most distros have caught up in terms of usability, and now it just seems like Canonical tries to do things differently for no benefit. (See trying Unity, but switching back to Gnome, or trying Mir, but switching back to Wayland.)
Yes, it’s multi platform. You can make websites, Android/iOS apps, Linux, windows and Mac binaries.
The gimmic is: on desktop, it runs on a very barebones, chopped down chromium based runtime, instead of using native libraries. So, it’s resource consumption is a lot less than Electron, but higher than native apps.
Lemmur is Flutter based.