The use of an idiom doesn't change the definition of the words.
Probability is not causal. Just because something is "highly likely" does not mean it is true.
Then you don't understand what an idiom is.
"The ball is in your court" is an idiom. It doesn't mean you have a ball or a court.
When an American says "chances are" he's not talking about probability. The closest replacement for a non-American would be 'probably'.
Yes, he is. "Chances are" (or, equivalently, what the headline actually says, which is "odds are") is an idiom that means "it is more likely than not", or, in probabilistic terms, "there exists a probability of greater than 50% that..."
More to the point of arguing about the meaning of an idiom, in this case, Musk has actually been a lot more specific and quoted a specific odds ratio of 1 billion to 1, or approximately 99.9999999%.