If there really were a contrived scenario where exactly two people are allowed to contribute, the full 6k does need to be provided, and one person has no means

If there really were a contrived scenario where exactly two people are allowed to contribute, the full 6k does need to be provided, and one person has no means to exceed 5999 then the split is still fair in some sense (and is the only "fair" allocation in the sense described by Shapley -- efficient, symmetric, and linear). What you're seeing in this hyperbolic "counter-example" is that the assumptions of that model don't hold; the person providing 1 rather than 5999 is probably very replaceable and would be willing to settle for much less profit due to market forces, the person providing 5999 probably has the means to at least temporarily acquire another 1, the deal probably wouldn't fall through in the first place if it were for slightly less than 6k, and so on.