Interestingly, they've made only the source code open-source. They require a license agreement to download releases. See for example: https://github.com/wixtoolset/wix/releases/tag/v6.0.1
But it can still work. in a company, as an employee, you can say a tool needs to be paid and get your company to pay even if there are binaries available elsewhere.
I work for a company that sells open source extensions. It would be pretty trivial and legal to remove the license checks, but companies just pay.