When people say Valve doesn't have a monopoly, they usually mean they don't engage in anti-competitive practices (like making exclusivity a condition for publishing on their store, cough cough).

Actually, Valve's recent moves represent what free market capitalism should be about - when competing stores started to appear, they instead made massive contributions to Linux gaming and appealed to right-to-repair advocates with the Steam Deck. Now both of those demographics are suckling on Gaben's teats, myself included.