Both.
Mojo standard library is already open source. Mojo at the moment does not need a runtime (but if it ever needs one it'd get open sourced). My point was, Mojo as a whole, as a programming language & a reference implementation, will definitely get open sourced.
MAX itself is a bigger beast to work with, and I am out of my depth to talk about it. I think it'll get open sourced as well, just the timeline might be different (shorter or longer, IDK).