So then if I do "mix deps.get" to fetch elixir libs, will that work? will it be able to compile files that are outside the sandbox?

if mix can work without sudo|root it will absolutely work in an "immutable distro" on the other hand: this particular immutable distro may not have all the c libraries BEAM/Elixir expect in that base and while SilverBlue does let you add to the base this doesn't sound like it will. So it might take some effort, hard to say at this point, though you can always add to your PATH