Their FAQ says this:

  Over time we expect to open-source core parts of Mojo, such as the standard library. However, Mojo is still young, so we will continue to incubate it within Modular until more of its internal architecture is fleshed out. We don’t have an established plan for open-sourcing yet.

I read this as:

"There are parts of Mojo that we are not planning to open source"

Which makes this a non-starter for me as well