M is "thousand". MM is "thousand thousand", which is "million", and a fairly common shorthand. I haven't really seen it, but its pretty clear then that MMM is meant to be "thousand thousand thousand", i.e., billion.

You could have also written it as "$10B" and 100% of people would have understood what you're talking about without an explanation.

Why didn't you write 500M cars as well? Could it be that it's a stupid convention?