Even if the USA manage to change the government, without troops in the ground to control it, that also do not match a invasion,
There were troops on the ground. They just left already. Presumably what Polymarket intended the word "invade" to mean is "occupy", i.e. invade the country and then continue to keep troops there long-term to stabilize the country. However, as a general matter of law, the word "invade" does not require occupation, but rather occurs when you have two factors: entry and enmity.
Entry: The presence of foreign troops on another country's soil. Clearly, U.S. troops were on Venezuelan soil, because they captured Maduro.
Enmity: This means the intent to act as an enemy of the the country in question. Clearly U.S. troops had hostile intent towards the government of Venezuela, because they arrested its leader. And as a general matter of international law, having hostile intent towards a nation's government is generally considered to be hostile intent towards the country (except if the actor is part of that country, e.g. in the case of a civil war). Therefore, this military action appears to involve enmity.
Now folks can certainly argue that the intent was to rescue Venezuela from that government, and argue that the troops did not intend to act as enemies of a country, but only of a specific individual, but as the head of state, this argument is problematic at best.
So the question is whether Polymarket provided a definition for "invade" ahead of time that is different from the traditionally held legal definition, in which case they might have ground to stand on, or failed to do so, in which case they will likely be forced to pay out.
The definition that they provided as their justification — “US military operations intended to establish control” — appears to still be problematic for them, as the clear intent of that military operation was to establish control. Our president literally said that the U.S. would run Venezuela until a new president could be elected. So the control over Venezuela being temporary doesn't change that the intent was to establish control. Additionally, he made it clear that part of the intent is to restore U.S. corporate control over Venezuela's oil fields, which is also a form of establishing control, and is likely *not* temporary. So whether you use the legal definition or their own, Polymarket's conclusion seems dubious at best, and potentially outright fraudulent, depending on whether they have skin in the game.