Yes, I suppose. Remember people are parts of various nations. Your "national security threat" might be a "national security opportunity" for someone else ;)
Leaking classified information is a crime, and I don't see why foreign adversaries can't monitor Polymarket to get advanced warning of military actions. A less corrupt US government would ban betting on matters involving national security.
Also I doubt any adversary worth their salt needs to monitor Polymarket.
I remember reading once that when the US was working on the SR-71 Blackbird out in the desert the Soviets could read its cross section by using satellites to pick up the thermal signature its presence on the ground left behind. State level actors aren't doing pedestrian stuff like watching pizza deliveries to guess what the Pentagon is up to.