There isn't a single 401k that Americans use. 401k is an index fund, an investment vehicle. I'm sure many included SpaceX but plenty did not.

And as an individual you can't really "opt out" of individual stocks in the fund. You could roll all of your investment into a different fund that doesn't include the stock you don't want, but I don't know of any broker that allows people to pick and choose which stocks they want to use in their 401k. You give them money, they manage the fund, that's how it goes.

Oh, I hadn't clocked until now that 401(k)s are a pension fund. I guess that means they are basically the same as superannuation.