This is a false dichotomy.
For years Golang was developed in the open but strictly moved on the vision of its creators rather than being "community-driven". Many other venerable open source projects don't involve the community in serious strategy discussions. The community mainly acts as a bug finder/fixer. Mojo could do the same: be open source but choose its own priorities internally.
I'm guessing that Mojo is still looking for a monetization strategy. Keeping important things proprietary in Mojo at this stage helps I'm sure (nothing wrong with that).
But I feel the era of proprietary programming language play is over. Unless you create some hardware (which the Mojo guys don't) it's going to be tough.
Release the source, but don't take code from external contributors. Take issues and discussion instead
Translated from corporatese it means "it will never happen".