I'm glad you enjoyed the game, and thank you for the feedback! :)
I'm writing a detailed postmortem for this project that will go into details about the tech, but in the meantime, if you're interested, you can read the postmortem for my GBA game that uses a very similar approach: https://dreamnoid.itch.io/aereven-advance/devlog/800104/aereven-advance-postmort...
For the rendering, it depends on the build. For the longest time, the web build only used the basic Canvas API but it finally outgrew it and I had to replace it with a WebGL renderer. The Windows version simply calls my "Simple Platform" (SPF) library that was already powering my previous engine (it's built on top of SDL2 and OpenGL). Likewise, the map editor is the same I used with my previous engine and is part of an integrated suite of tools I used to make the content of the game and compile it for the C engine.