A lot happened since the cart release. The cart was released two years ago with version 1.0 which was an NROM game, had only one character, only one background tileset and no netplay. So the version on this page is a fully working game, far more complete than the cart version. It will continue to receive updates as I improve things and add contents.
Actually the main mapper for version 2 will be the RAINBOW mapper, which is not yet available but will have an integrated WI-FI chip, allowing the netplay from real hardware. I also build compatibility ROMs for common mappers, so it works on emulators without RAINBOW support, the mapper 30 version is just that.
Finally, yes it is coded from scratch. I also implemented RAINBOW support in a fork of FCEUX for that. You can find the source code on github if curious ;) If you prefere a more high level take on interesting technical topics, the devlog of the game (here on itch.io) contains a lot of technical highlights.
I hope to have answered all your questions. Do not hesitate if there is still anything you want to know (I'll be more than happy to brag on any topic xD)
