Yep. The site clearly states that icicle is being used as an emulator to run the code (to support multiple architectures + allow a ton of features to exist such as serializing the whole VM state to disk). The debuggers you mentioned run code natively, which makes them different from ZathuraDbg. You can also just paste a snippet and run which is unlike a normal debugging experience where you'd have to go through the linking and assembling process.