Well the "expertise" is mostly just Python thats sort of the value prop. But yeah building an actual AI product ontop I'd be more worried about the early stage nature of Modular rather than the implementation is closed source.
More of a question of /will/ Mojo eventually be entirely open source, chunks of it already are. The intent from Modular is eventually it will be, just not everything all at once and not whilst they're internally doing loads of dev for their own commercial entity. Which seems fair enough to me. Importantly they have open sourced lots of the stdlib which is probably what anyone external would contribute to or want to change anyway? https://www.modular.com/blog/the-next-big-step-in-mojo-open-...
Feel like Mojo is worth a shoutout in this context https://www.modular.com/mojo Solves the issue of having a superset of Python in syntax where "fn" instead of "def" functions are assumed static typed and compilable with Numba style optimisations.