Take it one step further to illustrate the double payment, assume that the trucking company and Walmart had the same insurance company. The insurance company paid twice. Once through Walmart's employee health plan and again through the trucking company's liability plan.
Since there wasn't a single insurance company involved, Walmart's employee health plan is due reimbursement from the trucking company's liability carrier, or in this case, from the settlement.
BTW, what is missing from the story is how much the lawyers took from the settlement. In a just system, the Walmart health plan would have been in-line for payment before the lawyers.
Now I am interested in understanding how a fully self funded company differs from one who is not.