Russia doesn’t have the capability to project significant power to Cuba or Venezuela. And neither Cuba nor Venezuela have functional facilities in place to adequately host Russian forces.
The Russians will need to send construction teams ahead to build facilities and improve whatever airbases the locals let them use. Then they’ll have to transport everything they need from Russia to Cuba/Venezuela because neither country has anything useful let alone food or toilet paper. Then the matter becomes one of ongoing logistics to support these forces not to mention that the Cuban and Venezuelan governments will demand bribes from the Russians.
Were this threat made to me I’d respond by saying, “Go ahead. Let’s see you do it.”
The problem with uttering a threat like this is that it’s made from a position of weakness. Russia can no more afford to put men and weapons into the Caribbean than they can afford to put a man on the moon. And the moonshot would be easier.
Yet. Germany didn't have the capability to do anything while the French were rubbing their noses in it in the 1920s, but we all know how that turned out.