You haven't presented any evidence that Ford is accepting lower margin by selling to fleet. You haven't presented any evidence that GM is growing the truck margin by focusing on retail and ignoring the 40% of the market that Ford and Ram are not ignoring.
The only thing we know is GM is still spending more on incentives than Ford to achieve less sales in fullsize trucks. That implies exactly the opposite of the theory you keep floating.
When you are a volume player in the segment, you don't maximize profit by not selling to the biggest buyers. That's a classic mistake. A volume player in an oligopoly market should only deploy the strategy to maximize marginal unit profit when you are production constrained and/or when your competitors cannot respond to your higher prices. Absent those condition, you are just bleeding sales AND profits. Fullsize truck market is a classic oligopoly with 3 dominate players each holding roughly 30% of the market. GM is not production constrained on fullsize pickup truck and Ford and Ram are more than capable or taking away sales from customers that GM is not interested in selling. In this situation, you can't maximize profitability by restricting volume... you are just spreading your fixed costs over fewer units.
This is different from luxury cars or midsize cars where it is not an oligopoly and GM is not a major volume player. In those markets, GM can restrict output and try to maximize profit. In another word, GM can find a niche and try to stay out of the way of dominate players in those segments. In fullsize trucks, GM can't stay out of the way of the dominate players because it is one of the dominate players.