Looks like the underlying project is not open source:

"WebVM is based on a CheerpX build that we host. We encourage users to deploy their own versions of WebVM, but we don’t currently allow self-hosting of the CheerpX engine itself. This public build of CheerpX is provided as-is and is free to use for technological exploration, testing and non-commercial uses. If you want to build a product on top of CheerpX/WebVM, please get in touch:"

https://labs.leaningtech.com/blog/mini-webvm-your-linux-box-...