The only guess I have is because it's the default interactive shell on macOS, while bash is probably more common on GNU systems.
But that also doesn't make much sense given that this is about non interactive scripts.
To be honest it's kind of crazy that for all the work that's gone into nodejs, it either doesn't have, or people don't know about, basic functionality that these examples are running a shell for.
The manual page for mksh also mentions Android in the introduction for those who do not understand the role.