Earlier quoted context omitted.
From their FAQ https://docs.modular.com/mojo/faq.html "Will Mojo be open-sourced? Yes, we expect that Mojo will be open-sourced. 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 yet. Why not develop Mojo in the open from the beginning? Mojo is a big project and has several architectural differences…
The problem with this is that you don't know what the licensing will be when/if they open source it. So you could start using it now and they might change their mind and choose not to open source it or it might be a restrictive license.
(A canonical project can be steered in a way that makes it impractical for you, and forking is often also impractical.)
For now, I'd treat it as closed source, which is a non-starter for investing in, when I can accomplish the same in open source ways. And there's no sense in giving away the open source uptake benefits to a company when the software isn't open source.